I don't want to seem like a salesman for them, but the guys at www.kgtrimning.com have B20 con rods with the old 6-bolt sized big end and a 24mm sized (new, metric) sized little end (part # 350421 for 200 Kronor/~$20 each). This would allow you to use stock B21 pistons in an overbore rebuild (about $52 each from them).

Although peering through their catalog again I also see 92 mm (B21 sized) pistons with 22 mm (old 6 bolt con rod size) wrist pin holes as well for 575 Sw. Kronor (~$57). I guess the advantage to getting the rods and using the B21 pistons is the abilitiy to use B21 oversizes (92.5, 93 mm) in future rebuilds - but I don't know if you can actually take a B20 block (originally 88.9 mm) out to 93 mm safely or not.

To apply this to a B30 you'd just need to buy two sets (or talk them into selling you a set and 1/2) - then balance the entire set carefully.

For about $1200 they rebuild a short block (block, crank, pistons, rods, all bearings, oil pump) to overbore specs. The catalog is in Swedish, so that might be exchange or shipping your block to them. Don't know about the shipping costs on something so heavy and large, though...