Try your local Chevy dealer.
# 3899006 is a four speed crossmember for a camaro. they might still have them. I do know they still have the big block turbo 400 crossmember, but it will not fit your trans, the TH 400 is longer.
Nova up to 1974 uses the same crossmember. Firebird may use it too.
I think you would be better off replacing the whole sub frame rather than trying to weld in a new crossmember. To weld in the crossmember you will have to be sure it is in the perfect position, which I don't think you can do with confidence without a jig. What is wrong with your current crossmember?
70 Camaro 383ci
69 Camaro 385ci
91 Camaro RS V8 (305 TBI)
Link to my 70