I know I've posted just about every question about transmissions known to man over the past few weeks but Im still having issues.

So here we go, my DD is a 75 F150 2wd, with a 390 and C6. The parts truck is a 78 f150 2wd, with 351M and C6. Long story short again DD wont go into 2-3 when in (D) and I can Manually shift it to 1st and 2nd but it wont go into 3rd. Yes Reverse and Park work.

I know the C6 transmission cases are different between the two trucks. Now my wife asked me, " Well why dont you just swap the guts in the transmissions." And I said to myself, Hmm I think shes on to something.

So here I am with the million dollar question. Can I put the internals from a 351M C6 into a 390FE C6.

 

Yep, just not the torque converter, but I wouldn't go to the trouble to swap parts without a rebuild kit anyway, its not worth the time to have some rubber seal rot and start leaking three months after you swap internals.

 

You have to completely disassemble the trans to swap the tail shafts so you might as well just rebuild what you have. Do not reuse the TC from the bad trans, there is no way to flush one out and be sure to flush your lines and cooler out real good.

 

Yup, you can pretty much swap all the parts. But, like thekingofcows and mark a. said, you might as well throw in a rebuild kit. You're going to have to tear it down regardless, at least you'll know what you've got.
With the transmission not shifting into third, the three areas I would pay close attention to while the tranny is torn down is the vacuum modulator, the valve body and the governor assembly. Any of these three could cause the symptoms you describe.
And X3 on replacing the torque converter and X2 on flushing the cooler lines, a lot of people overlook that one.

 

thanks for the help got a recommended rebuild kit and TC opinions.

 

I get most trans parts from Oregon Performance Transmission.

 

I got my rebuild kit from rock auto. I've been very happy with them with everything I've bought.