Would i be able to mount a th400 to a stock g-body cross-member.

 

If you have a 200-R4 trans now it will bolt up. You also need to make sure the 400 trans is a short tail.

 

nope its a long tail! already planned on taking the drive shaft to the shop to get cut though. So i know its made for the 200-r4 but will it fit the th400? i also have a cut up make shift cross-member that held in my th350 will this one do if the stock one doesn't?

 

You will need to extend the frame, and move the stock xmember back.

OR
if you want to be ghetto about it, a 73-77 A body x member will ride inside the C channel part of the frame. You could drill some holes and put some bolts in that way, but it won't be as strong. Good enough for a daily driver, but if you're going to race it, it will most likely fail tech.