Transmission tunnel help

[Eternal]

Hello all.
Have been doing research and im trying to work out what to do with the tunnel when i make the chassis. Seeing as there are quite a few sierras out there with MT75 gearbox;s should i just make the chassis to MT75 size reglardless of what donor i will eventualy get?
My question is... my endgame plan is to install a 13b engine and gearbox from a fc3s into the chassis i dont suppose anyone knows what tunnel size would be best to make this fit?