1968 C20 Pickup Trans Swap Questions.... 4L80e

[StreetRodBrent]

Hey everyone,

I have a 1968 GMC C20 single cab long bed pickup truck. Purchased last year, was a California truck so almost rust free. So I have built a 357 (350 bored .40 over) with cam, heads, and flat tops and a victor junior. Making a tad over 400hp and ready for nitrous, electric fan, fuel cell, long tubes, line lock... and right now still hooked up to the stock 4 on the floor SM465 trans, and a tow truck rear with horrible gearing for anything but towing(3. or 2. something?) . Yes, not the setup you would think of in a 4800 long bed truck and go ahead and call be crazy but hey.. Its fun and thats what its all about!. But for that reason, next is a 4l80e trans swap as well as a 3.73 rear end both in the garage and ready to be mocked up, cut, grind, build, and put in. Questions are... Cross member, and tunnel modifications. Any input on where to buy, if i should fabricate the cross member out of the truck to go in, do I need to replace the tunnel, and also do I need to pull the tunnel out to remove the SM465 4 speed in it? (Was a hard time just to do the engine swap.) And then any other input on other details or parts. Planned on swapping the two peice shaft to a one piece if possible or shortening whats there and picking up the high horsepower two piece shaft carrier bearing, then hurst shifter, good tq. converter, TCI trans controller and trans cooler and all. Very skilled mechanic and ready to do it but would love to hear some input from some of y'all on how I should go about some things. Thanks!

Brent

[StreetRodBrent]

Ttt

[StreetRodBrent]

ttt anyone could even point me to another thread about the subject?

[wbc409]

Your truck being a stick should already have the high hump floor, so I don't believe you should have to modify the floor. All you need to find is a cross member from an automatic truck. You could get away with just shortening the front shaft. TH400 uses a slip yoke and a different spline count than your manual transmission. If you use an inland empire billet hangar bearing the rear shaft may need to be modified with a sliding tube added. You will need to remove the manual transmission mounts as welll

[Andy4639]

I had the 6.0/4l80e installed in my 71 LWB truck. Used the stock transmission cross member and the tunnel was fine no mods at all to it. The transmission is about the same as the 700r4 I had in it before. You will have to have the drive shaft shorten and what do you plan on using to control the 4l80e it works off the ECU and for a speedometer it's electronic.

[StreetRodBrent]

Okay cool, so as far as the tunnel im glad to know im safe. Driveshaft will be a figure out as I go situation for what works best and then I have been looking at buying the MSD Atomic trans stand alone controller and then I believe that has a digital read out for the speedometer but I planned on doing the electronic to mechanical adapter setup anyways. Unless I need a different controller to do that...? But Also has anyone pulled the 4 speed sm465 without removing the tunnel or pulling the engine and just chopping the trans mount out to drop it?? When I had pulled the junk 350 out of it to drop in the bulit engine I know it seemed barely possible to even shake the trans around sitting in the truck because of the high sitting top load shifter on it... So hoping chopping the trans mounts out will let me come back enough and straight down to clear the hole in the tunnel with ease...

[StreetRodBrent]

And I have the cross member out of the c2500 I pulled the 4l80e transmission out of and the frame width difference is only a inch or so off so I should have plenty of play room to cut down and fabricate the cross member to bolt in on my 68 with the trans. May have to weld some drop brackets or even spacers but shouldn't be much of a challenge for myself.

[StreetRodBrent]

Heres a picture of my ole' girl too because im sure you guys would love to see what Im working with.

[wbc409]

If you use a non original cross member for the transmission, make sure you check your drives haft angles. Don't forget to check for correct orientation of yokes on the shafts. If you don't do this you could have a nasty driveline vibration.