No real experience, still working on a 235.
The maximum CR is very limited in the 235 by the chamber design - except for a $$$ custom piston even getting 9:1 takes some work.
The GMC has very different limits due to the more conventional chamber shape, but again a domed piston will be needed for anything big.
How much cam? Above perhaps 230° @ .050" the idle quality begins to decay, the power band moves up, but 235 peak power is disappointing, 240°+ will add peak power and need to be driven like a Honda, no more "slip the clutch to get away at idle" that made them good truck engines.
Vizard and others suggest that an engine with limited intake valve area should try a much narrower LSA than any available cam, like 104° or less (108° is a hot cam now). Big $$ and may produce little.
I doubt that a 250 HP 235 is possible. The head design is a dead end. This means that it needs either a 5-speed or a loose torque converter not to be a misery to drive in traffic. Or make a Rajo?
The GMC (H head) is much better in port size and port design.
Either one needs a tube exhaust but, with siamese ports, real sonic wave tuning isn't as effective as on a "modern" engine (Slant 6). For intake my first choice would be fabricating 3 separate (but alike) runners, 1 to each siamese port, ending in a HD6 etc. SU. Large enough for 250 HP in the XK-E.
Much work can be done on windage, pumping loss, case pressure management that has never been tried but can be "borrowed" from Jaguar, Toyota, etc. (an L6 is an L6 to some extent). Santucci doesn't discuss in his 250/292 book - but I'm sure he's done stuff.
