[MKS427]

I would sit in them also, before you make a decision. The Factory Five interior is considerably tighter than most. I’m 6 foot tall with size 13 foot and it’s a tight squeeze for me. The ERA, Superformance, and Backdrafts are considerably roomier both in the cockpit in the trunk.
If you were considering tracking the car, then I would lean towards Factory Five Racing complete kit, as it is set up better for the track.
Regarding the engine, keep in mind not every manufacture supports every engine. My personal choice is a 351 Windsor based Dart block stoked to 427, can go as big as 468. You can achieve more power than a 427 FE for about 6K and 140 pounds less.