TH350 interchangable crossmember for 68' C10?

[iGoTzSTYLE]

Need a lil direction, I bought a 68' C10 Fleetside Roller and now have the engine and trans (383 and TH350) at least halfway in. Had a bit of trouble with the engine towers being wrong and have finally found the correct sbc towers but now have to find a trans crossmember that will work correctly with the TH350. The truck was originally a 3 on the tree and due to the towers that were in it I assume an inline 6. What year C10's do I look for that may have an interchangeable trans crossmember? Any help or advice on this would be most appreciated as its my first build and need all the expertise I can get.