I was wondring if a tranny crossmember from a 66 chevelle will work in a 66 el camino? I have a bunch of spare parts left over from my hardtop that I will be using for this truck.
I aslo have two different crossmembers. One is tubular, with two humps for exhaust built in, flkat ends- the other is flat steel, stamped with the humps and flat ends. Is one for an auto and the other a manual?
I plan on using a 350 with a 400TH. What kind of changes will I have to make for the 400 as compared to using a PG or 350TH? Thanks for the help!

The cross member should work fine only you'll have to move it back I think 2". There is no difference between the cross members that were in Standards and Automatics. The Crossmember that came in a Car with a 350 Thd is the exact same as one that came on a car / truck equipped with a Muncy 4 speed, the mount points are the same place. The 400 turbo and the 350 have a slightly different mount point as do the 700R4s and some others.

Other things that will matter is the Prop shaft length. It is probably going to have to be replaced or cut down some.

Have fun with it!