Factory motors?? Second Gen 12Valves are really the best, considering the P7100 IP pump, and the piston lift pump, and with the manuals being a higher rated pump. then the good old VE IP pumped 12 valves. (got one of those)
However I think the ulitmate would be a 24valve with the P-pump conversion. They put more valves in it for a reason!
for easy and cheap HP, the VE equiped 89-93 12 valves are the cheapest to instantly turn up, how does almost 100 HP for FREE strike ya? after that the price goes up quickly. Reliability? I have 336K on the original motor, a friend of mine has 450K on an original motor. some have more. It all really depends on driving habits, and maint. habits.