Printed circut panels gauges wo tach VS gagues with tach, and bed differences

[BSR63]

New guy here asking the age old question in reference to the factory tach. I picked up a 78 HD gauge housing will all the gauges and tach to compare it to the gauge housing in my 76. What I noticed is the tach is separately wired, and my oil pressure gauge is mechanical. So here's the question, and my simplified reasoning.

1. If you were to purchase a printed circut panel for a 76 with gauges and tach, would it work minus the mechanical oil pressure?
Now for my simplified reasoning..(LOL). I cannot see GM hanging untold wiring harnesses on the assembly line for use, when it would be simpler, and more efficient to utilize the same harness and just alter the circut panel and applicable gauges, warning lights.

2. The bed on my 76 is trashed, rust, holes, a complete waste and to far gone to repair. I was told to find another which fortunately I was able to. The bed has fuel doors and I need to order the applicable hoses, filler neck, etc... How can I tell the difference between the years or does it matter as long as I get the appropriate unleaded neck?

Appreciate the help.
To CptKaos, thanks for posting the electrical diagrams.

[rich weyand]

My '78 has an '86 bed with fuel doors.  Some IPO connected this bed jury-rig fashion to the '78 tank, using radiator hoses!  Not good.  They are not fuel-capable, and were cracking and rotted.  They tried to modify the existing fuel filler neck by cutting it off short and clamping these other hoses on.

The problem is that there are several differences besides just the fuel doors.  One is that the fuel cap is at an angle inside the fuel door, where the fuel cap on the earlier ones is flush with the sheet metal -- basically vertical.  Another is that there is a difference in the diameter of the fuel filler line.  The later models are 1/4" bigger in diameter to make them easier to fill.  This means everything is different, including the tank.

What I ended up doing, finally, to get it all correct is to use all '86 parts ('82-'86 is the range) to put in dual tanks.  So I got '86 tanks, fuel filler hose, fuel filler vent hose, fuel filler neck, and the fuel filler housing.  Everything is available from various on-line vendors.

But what you need to know is that nothing is compatible between the fender-mounted filler setup and the fuel door filler setup from the cap all the way to the fuel tank mounting brackets.  It's all different between your existing truck and the new bed, and the fuel system components should be compatible to the model year of the bed, not the truck.  I think I reused the existing sending unit from the one tank, and I got a new sending unit for the second tank.  Those were compatible, but if you look at parts listings, there are two different numbers.  Not sure what's up with that, but I only remember getting one new one.  If you need to know, I can go back to the receipts and look.

For all that, it's a sweet setup to have an earlier truck with the fuel-door bed.  I like mine, now that it's all sorted.

If you have any other questions, fire away.  You can also PM me, or email me at weyandATrcnDOTcom.  No sense re-inventing the wheel when I did this all already a couple years back.

[Captkaos]

The wiring changed 3 times between 73-80, and the cluster was changed on this same pattern.  For plug and play, you need a 76-77 cluster, otherwise you need to rewire the truck to add the electrical wire for the oil..
The tach is a separate cluster so adding it isn't the problem, it is the oil sending.

I would have gotten the related parts from the donor.  I have done this with swapping tanks...

[BSR63]

Thanks Rich. In reference to the new bed, I was going to order all single year parts, i.e. all 84-87 tank filler parts, filler neck, hoses, fuel cap, etc... Since the 76 tank sending units are good and working, I was just looking at the fuel tank to the bed fuel filler door. Do you think that the sending units will not work? Guess I should keep a log of all changes in parts vs years.

Captkaos. Thanks for the info. I was thinking that a new circut panel with the applicable housing would work since I have two, one for the mechanical oil pressue gauge and one for the tach connection. As things are progressing, I am thinking that a 3" or so tach on the steering column may the simplest solution. But the factory set up is so much cleaner.

[rich weyand]

I think I used my '78 sender in one of the two tanks.  I only bought one new one, and I bought one to match the one I had.

You're on the right path, though, getting all the right parts for the bed year for the tanks and filler neck parts.

Also, use the motorized Pollak 42-300P tank selector valve, not the solenoid one.  Same mounting holes and hose hook-ups, but much more reliable.  Not cheap either.  This is probably the best $$ out there:
http://www.partdeal.com/pollak-motoroizedvalve-kit-42-300p.html

[BSR63]

Rich, everything is working presently. The fuel switch, sending units, etc... someone replaced the swithcing unit as its a 6 (?) port design and currently only using 3. But with the older model year emissions I guess thats all you need.
You had mentioned earlier that the fuel tank could be a smaller diameter than a newer neck? Thinking that a replacement fuel filler hose would not fit and I may have to cut a newer neck down to match a smaller diameter hose.
I was going to purchase the entire set up from  , and hopefully they have a tech dept to answer the 76 tank to 84-87 adaption.
I thought I read in the threads that there were a few more differences in the beds but I guess it was just the round exterior fuel filler to the square hidden. Im clise to pulling the bed and I will start on the fuel fillers then.
Appreciate your help.

[rich weyand]

I got two new tanks; the old single one was original.  Not sure what kind of adapter you could use.