Your correct the buick has a different bolt pattern. The 4.3 is as condusive to boosting as is the sbc, basically the same engine minus 2 cylinders. The even fire crank and the odd fire crank both work fine. They make an even fire steel aftermarket crank, I think it's LA enterprises. The odd fire is cheaper in steel. You can install or at least I've seen 4 bolt caps installed on the 4.3 block, don't know how it worked though. Pistons are the same, rods are basiclly the same, just narrower by .050 or so, easy for a competent machine shop to narrow properly. If you go with the money is no object theme, they manufacture rods for that application.
I'd like to see it work.