Can you convert an SC400 V8 to the Supra TT 6-inline as easily as you would with an SC300 that alrady brings a 6-inline ???????????

Yea with new motor mounts and some fabrication yes.

and i think a different subframe........

I thought it was a direct fit in. Only thing is the supraTT wiring harnes will not directly fit the fuse box or the body harness plugs.