I'm not an expert but I would go with the 4.125 x 4.00". Higher rpm motor. You may lose a little torque at low rpms but your pushing a lite car. Watch what trannie, tire size and rear end gears you pick. Picking the correct components and you will have a great street machine. Quick off the lights and great cruise speed (rpms).
I'm a small block guy but I have 8 or 9 FEs in the shop. One of these day I would like to build a GT 40 with an FE.
This is going to be a very informative thread
Dwight