Motor, Transmisson swap questions

[MedicineMan]

FNG here, I've searched for the answer to this one and I'm still unclear. I have a 68 longbed, that was a 250 six / 3 speed manual. Should I or shouldn't I use different motor perches (towers?) to go to a 350? In some posts I searched, sb v8 perches are mentioned but I don't find them listed with any of the vendors.

Also my motor is a modified 350 sort of a package deal that i lucked into. I haven't decided on a trans, either turbo 350, 400 or 700r4. Does having a heavily modified 350 make it nesessary to go with the turbo 400 or can the 700r4 or turbo 350 be made to stand up to something like that.

Thanks. I introduced myself on the board in September but am just getting active on my truck this spring. I cant tell you how much help this site has been already.

I've got my frame painted, Early classic drop and disc conversion, front end rebuilt, and engine ready. I've just got to figure a few things out and I'm go for drivetrain install.