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The older B20 rods are made from better material & heat treated. If you have to, ARP have bolts, but they are never required. These come with the 6 bolt crank with SAE bolts.
The downside of the late rods which are grossly oversize, but poorer quality and weaker, is they are very heavy. The matching pistons are also heavy, as too are the B21 pistons. The ipd trick of selling B21 pistons and a big bore headgasket for the late B20 motor is bad. You loose a heap of power stoping and starting the reciprocating parts. 90% of people will tell you the B21 piston & rod 2127cc bigbore is a step up from their old B20 because they are comparing an old worn out motor to a new one with higher compression.
Block: They all go to +.125", but some will end up stinkers. I like the one with extra pieces above the tappet area, these appeared with injection. The last blocks which featured metric main bolts I put aside because the bolts look pathetic alongside the old UNC ones. If you have to use one, tap the bolts holes up a size and buy a fatter set of bolts.
Your head: When you say 'modified', by who & what was done? If you have no flow bench figures, I would bet it flows the less. The upside of an E or F head is the big intake valve + the exhaust valve is suitable for unleaded gas. They are easy to get to a 'mediocre' stage, good with twin 45 DCOE's or the injected manifold, but they don't flow as well as a properly flowed A or B head. Also note that the newest A, B, E, F head has an improved exhaust port. These started to appear around 1973. If you are staying mild the late model exhaust port needs just a 3-angle valve grind, a tiny blend into the big throat cut, and a very carefull curve on the SSR. E & F head intakes need just a 3-angle and a mild blend in the pocket for cam lobes up to .300". A 'big' cam requires a lot of widening of the SSR to slow the inacoming air and help it turn. What head depends on what you are doing and your budget.
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