go with the 4.15 stroke i have done 3 of them now
and they are very reliable and rev all day long
one with 915 heads 2.14 8.18 valves comp hyd cam rpm 440 850 demon made 450hp @ 5000 and 550 tq @3000 small cam
one with rpm heads 6pk solid roller made 600 hp @6000 600tq @ 4000
one with victor heads solid roller indy dp 1000 cfm hp carb that i have not fired yet
the 4.15 is a true street strip combo that works good
if it aint broke don't fix it