From what I can tell, there has to be about 30 hp difference between a good modern iron block and a Shelby aluminium. I have had two almost identical engines on the same dyno..........the iron made 920 hp, and with bigger valves and about 20 more cfrm through the middle of the lift, the aluminium engine made 900 hp with a top-of-the-line vacuum pump. There was probably another 20 hp in the aluminium engine due to exhaust flange mismatch with my dyno headers (have checked it before), but the vacuum pump is worth something on the aluminium block, and the better head flow is also. I think an iron block with the vac pump would show me 950 hp on that combo, and the alum 920 hp if I built a set of headers that fit the exhaust port correctly. My opinion is 30 hp iron vs. aluminium with everything else a constant..............but that is at the 900+ power level. In a 600 to 700 hp street engine, it is not that much, speaking linear terms, but there are other things to consider on the street. The lifters will bleed more when hot, and the mains need to be "adjusted" for expansion on the aluminium........on the street or the track. Power vs. weight, it is ALMOST a wash for a drag car. The power would almost be offset (within a few hundredths) by the weight difference, but the car should "work" better with the weight off the nose. In a road racer, with a dry sump, I say aluminium all the way. On the highway, I lean toward the iron from the long term, trouble free use aspect. JMO.
