Transmission issues

[67Mach1]

I hope I get this all correct. I have a friend that owns a 66 Long bed Fleet and a 62 Long bed step. Right now the 66 is almost a runner but has a destroyed second gear in teh tranny. We were going to put a tranny from my 70 parts truck but realized yesterday they are not the same. Mine is over 21 inches in length and the stock (we believe to be stock) one on the 66 is only about 14-16 inches in length.

This brings me to my question. Can this 70 transmission be installed and simply shorten the two piece drive shaft or Would it be better to pull the transmission from the 62(which is also longer) and simply swap out the two piece driveshaft from the 66 for the one piece in the 62?

Thanks in advance.

Thomas