3spd trans to TH350 trans swap??

[68shortfleet]

Ok, i could not find this in the archives, but i am sure it has been answered before.
I have a 68 swb v8/3spd truck that i want to install the th350 into. I have the entire cab(steering column/linkage), all linkage brackets, and a crossmember for this conversion removed from a 71 c-20 lwb. This project truck did not have any motor or trans installed at purchase, so i have no idea how it came apart. It looks like there is a crossmember(riveted to frame) that must be removed and the new crossmember installed. Is it really that simple or is there another crossmember that goes in where the old one comes out as the auto one is much further back.
Hope this didnt confuse anybody.

Thanks
Jason