I had the same dillemma last fall and was wondering the best course of action to go. After lots of great advice from others, I went ahead and got a fresh set of long blocks and am putting them together with the intake, blowers, accessories, etc. that I had on my existing BBC. That way I had my old set of long blocks to try and recover some money out of. The idea of rebuilding what I had was costing the same money, but this route, I got somethign a little bigger, got a few more inches, and had a set of long blocks to recover somethign out of.
which reminds me, I have a set of Gen V 454 4 bolt long blocks that are being pulled out of my cig next weekend. 525sc's, they are great motors if you wanted to start with a another set of long blocks. email me if interested, they are posted in the for sale -> engines section, 2nd or 3rd link down. I will make you a heck of a deal on them. Mark