3 speed to 4 speed swap I need help!

[sampsonjoseph]

i have a 73 gmc 2wd truck and it has a 3 speed in the floor.  Someone changed it to the floor before i got it.  I want to put a 4 speed out of a 72 2wd chevy truck into my 73.  i can also get the drive shaft from them too but im confused on the carrier bearring is mounted different than my 73. My truck the carrier bearring mounts up and the 72 it mounts down to a crossmember?  So how hard will this swap be.  And is there any customizing to be done?

[Blazin]

OK 72 and 73 are two totally different trucks.
(1) Are we talking about a 3 speed to a truck 3 speed, with a low gear? Counting the low gear would make it a 4 speed, or a car 4 speed that has four gears?
73 and up truck 3 speed is an SM456, a 72 truck 3 speed would have had an SM420. They are similar but not exactly the same.
(2) 73 and up have a longer wheel base than 67-72 trucks. Drive shaft will most likely be wrong length.
(3) The 3 speed probably has a smaller clutch as well.

[sampsonjoseph]

yes it is a 3 speed without granny gear in my truck know.  so even if the drive shaft wont work then will the trans still bolt up to the bell housing and still matchup to the cross member.  and is there anything else i need to look at?

[Blazin]

I know from your description of someone changing it to a floor shifter your truck is Saginaw a three speed .

Do you want to put a Saginaw, Muncie, or Borg Warner 4 speed in it. Or are you talking about an SM420 3 speed with low??

[sampsonjoseph]

I'm not sure what kind of trans is in there now but i know it is run buy linkages.  The trans i want to put in is a 3 speed with a granny gear.  I'm not sure of the type of it either because we haven't taken it out of the truck yet.

[sampsonjoseph]

i think the trans is a sm465 though.

[zieg85]

You will need the 2 piece drive shaft from a 73.  You will also need a high hump for the floor.  I don't know if the cross member is the same between the 3 and 4 speed.

[Blazin]

Not positive but I think 72 still had SM 420's. I have seen them installed into 73-87 trucks, not original. It can be done. The 420's are not as rugged as the 456's.
The cross member might need to be moved, the output of the 420, and the 456 is a fixed yoke. Your 3 speed should have a slip yoke. You might get lucky and be able to mix and match drive shafts. But most likely you will need to find one or have one made. As far as the bell housing I am not positive ( been 15 plus years since I swapped a 3 speed out for an SM 456 ) I think it will bolt to the bell housing. The 3 speed probably uses a smaller clutch. I would upgrade to an 11" or 12" clutch for the bigger trans. Your flywheel might have the holes for the bigger pressure plate, or you might have to get one that will accept a 12" pressure plate.
 You will need the center hump from a 73-87 SM 456 truck. Will have to cut your floor center out and screw the 456 one down.