700r4 to th400 swap

[87highsierra2500]

First off hello to everyone out there I will be looking forward to talking to some of you. I have an 87 gmc high Sierra v2500 and my 2nd 700r4 is going out. I have a strong tin 350, 4in lift with 35in tires and 4.10 gears and I got my hands on a th400 with a 208 t-case, same t-case I have now.I'm just wondering if there's anything I need to do or be aware of before I install it at work. Like is the cross member going to have to be moved or can it stay and what about drive shaft lengths. The 700r4 isn't the original trans it originally had a Muncie 4spd according to the vin and rpo sheet. Thanks, Josh