Mr. P266 has very valid points. I don't think a full girdle to the pan rails would benefit much. Like he said...cost prohibitive (may as well go to an IA-2 block if you're going that far).
Steve Barcak's setup is a whole different animal. Making in excess of 2000-HP with a nitro' blown/injected '59 389 block. He needs it and fabbing a custom pan is not an issue.
I think the smartest move, both from an economics standpoint, and an application standpoint, would be to take the existing girdle and modify it to anchor down into the outer bolt holes that would normally be used for 4-bolt caps. Perhaps not as strong as a 4-bolt cap (or perhaps it may be)...nonetheless it would be a cheaper conversion than to install 4-bolt caps and have the associated align bore/hone performed.
The reason I say "perhaps it may be stronger" lies in the fact that, with a 4-bolt cap, you have better retention of the crank and bearing over the 2-bolt cap, but the cap is still going to "wave in the breeze" fore and aft in the block at high rpm. A 2-bolt cap with this girdle tied into the outer bolt holes of the block on either side of the cap won't provide as much meat around the bearing as a standard 4-bolt cap would, but unlike the 4-bolt cap by itself, it would be preventing that "waving in the breeze" effect of the caps.
Horsepower, rpm, and main caps:
I've run 2-bolt caps in a 455 block, with ARP studs, cast rods, JE pistons, and a factory crank up to 575-HP and limited the rpm to 5600-5800. Never had a lick of trouble. Having the block filled to the bottom of the water pump holes may have helped as well.
I've run 4-bolt splayed billet caps in a 455 block, with ARP studs, BME aluminum rods, Ross pistons, and an Eagle 4.25" crank in excess of 800-HP (on nitrous) and limited the rpm to 6800. That block was filled to the water pump holes as well, and I never had a lick of trouble with it.
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