Inliners International Forums General Discussion Drive Train TH350 behind a 250 Inline

I am looking at a couple different options for my truck.
1. Going with a three speed manual overdrive.
2. Putting a freshly built TH350 behind the 250.

If I go with the TH350 would I need a different crossmember? My truck is a 66 C10 with a three on the tree.