I know what you are saying, but I already have a brand new GT35 sat on my shelf , the cheap option would be a "top and tail" as Mike R calls it, new shells, rings, followers etc, check all the tolerences, quick port and polish and a set of cams, and get MSD to check it, and fit ALS etc as I have a nice WRC ported throttle body etc and the wild ALS idle valve.

Medium option is fit the mechanical followers and cams etc, still on the T4 with the above work, just to stretch the power band a bit.

Expensive option is to fit the GT30/35 with all the external stuff, and go for 550+ as the car already has closed loop/ wasted spark etc so is well prepared for it.

Or leave the head ports small, and aim for Rods old spec, 500/500 which must be an awsome drive?