Can't install my TH400 due to a cross member right behind engine

[Dogeared]

Have a 65 C10 with a 350 and want to install a TH400. When I tried to mount the tranny the cross member behind the engine (maybe 6 - 8 inches) blocked me from moving it forward.

This was originally a 6 cylinder 4 speed truck. I am guessing the cross member is actually the tranny mount for the 4 speed. It looks like it is also a part of the frame support. My plans are to cut the cross member out and then mount the TH400. I have a new cross member that I will install when I mount the new tranny

Does anyone have experience with this and will it substantially weaken the frame?