Manual Transmission Suggestions - Please

[Duess]

Good Afternoon, Square-Body enthusiasts. 

New to posting on this particular forum, however not new to forums in general.  I am located in Pittsburgh, PA. and have been working on cars/trucks since I was a teenager, which is creeping up on almost thirty-years now. In 2017 I purchased two "box-body" or "square-body" trucks. 
- 1978 Chevrolet K10 (Custom Deluxe): 350c.i./4bbl/4 spd. manual - 78,400 original miles, all original truck. I drive this truck often.
- 1975 Chevrolet C10 (Custom Deluxe): 350c.i./4bbl/auto - Previous owner hasn't had it running in over 10 years, so it is lawn ornament for now.

Since I sometimes drive the '78 to work, which has a commute that requires highway speeds. I was hoping to get any suggestions or experiences of possibly upgrading to a five speed transmission?  Maybe try to find an MM7 transmission [four speed manual w/OD] that came in the '82-'87 model years?  Go a different route?  Just looking for some friendly suggestions from people who are experienced in any of this. I am sure there may be replies stating that it depends on what I plan to do with the truck in the future.  For a reply to that is I am not really sure yet.  If I decide to really convert the truck and restore it, I will cross that bridge when it comes. Going to let my daughter(s) decide what we should do with them as time goes on. I truly do not mind purchasing & swapping out the transmission or even the engine a few different times. Main reason I am even thinking of suggestions of different transmissions is because the clutch is pretty smoked in this one and will soon need replaced.  Therefore, just trying to decide if I want to keep the four speed in there or get something little more highway friendly. Cruising down the highway doing 55-60 MPH ringing the motors neck at 3,000 RPM is not ideal on many platforms.
Please, do not reply with "LS Swap" or some dumb hogwash like that.

Thank in advance for your time reading and responding to this. Much appreciated.
Take Care,
  - D

[1967KaiserM715]

Nv3500 or 4500, 3500 isn't the best for towing or heavy use, the 3500 can be found in any gmc/Chevy 1500 from 89? To I think 06? Maybe... it could also be found in the light duty 3/4, without the 5.7 engine, otherwise it was upgraded to the nv4500. The 4500 is a more robust trans, and is essentially a heavy duty trans, or an sm465 with an overdrive.

Getting a trans made after 96 means a center throw out bearing, as the early trans have the clutch slave on passenger side, and this interferes with the front driveahaft.

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[Duess]

Kaiser, that is some great information and exactly what I was looking for.  I am pretty solid on motor knowledge, but I am appreciative that you've given me the detailed years below. Especially since most clutches I've replaced over the years (can count on one hand) and transmissions installations have been ones that were just swaps for selected years for that vehicle model.

I am going to make sure to read-up as much as I can about these options you gave me.  I like the idea of the Nv4500, even though I do not plan on doing lots of towing or things like that.

Thanks again for your help on the transmission suggestions.

[Duess]

Did one to make one correction on my original post in this topic in regards to the mph/rpm I was seeing.

I was putting a good amount of trust in the 1978 technology of analog speedometer.  My excuse for that would be ignorance and the thought process of "since everything else on the truck pretty much works well, speedometer should be semi-accurate as well".  I assumed the speedometer was accurate within a percentage reason (like 7%-10% off). Not the case. I found out this morning using my 'Waze' app on my phone that when speedometer reads around 50 mph, I am doing 62-63 mph, and fluctuation gets worse from there.  Appears that around 25-30 mph is when my speedometer starts inaccuracy.

That being said, I am doing about 65 mph when my stock analog tachometer is reading 2,750 rpm.  This is just a correction of what I said above in my original post that I was doing about 55 - 60 mph @ 3,000 rpm

Okay, Happy Halloween everyone. I am off to read more interesting topics on this board.

[1967KaiserM715]

I believe Roundhouse(member here) has done a nv3500, and done some towing.

For the nv4500, you would likely need an Advance Adapters bellhousing if you wish to keep the manual clutch linkage, for the nv3500 you would need to get a hydraulic clutch system, as the bellhousing on a 3500 is not removable.

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[roundhouse]

I believe Roundhouse(member here) has done a nv3500, and done some towing.

For the nv4500, you would likely need an Advance Adapters bellhousing if you wish to keep the manual clutch linkage, for the nv3500 you would need to get a hydraulic clutch system, as the bellhousing on a 3500 is not removable.

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I have swapped in a NV3500 in a 76 K-10
To replace the 4 speed granny
It probably will require a transfer case swap as well
Depending on which case you have now

The 3500 will bolt to a 32 spline NP-208 that has the round bolt pattern on the front

I'm not sure which year they swapped from the figure 8 bolt pattern  to the round pattern

The easiest and cheapest way to lower the engine speed is installing larger tires

What size tires do you have now ?

33s will usually fit with no lift

[Rattler12]

 Put a 4500 in my 83 shorty C-10 454 BB. Works like a charm. You will need the AA bell housing though at about $400

[roundhouse]

I do have some tips for swapping the clutch

Buy some bolts that hold the Trans to the bell housing

Get em longer than the real ones
Cut off the heads and use a hacksaw to cut a slot where the heads used to be

Replace the factory bolts with the new longer ones

This will let you slide the Trans off and on while keeping it lined up with the splines on the input shaft