I'd spend the couple of hundred bucks on a block from craigslist, but that's just me. My '91 360LA was a craigslist find. $150 up in Reno. It had the 308 heads (one with a crack between the exhaust and intake seats. No biggie, another $100 for another set, where one was cracked. SO, yes those heads were both running fine with cracks, and NO I won't be using them. Is $250 a decent price for a block and a pair of good 308 heads? I put everything except for the block, caps and bolts in the shed. New cast SCAT internal balance crank $416 at summit, forged SCAT rods $250 from a member here, forged SRP pistonsand rings around $600, and machine work around $2k thanks to my HOA and I am not allowed to even change my oil, all means that a new (used) block is less than 10% of the cost. Hell, now I'm not even going to bother with those 308 heads, and instead going with a set of ProMaxx CNC Shocker 185's. Unless I can see some numbers from IMM down in Indio on their new import aluminum heads, I'm pretty sure that I won't be disappointed. Now the cost of the block drops to around 5% of the build.
Do you really want to cheap out on the build?
edit:
I forgot, add in a cam for $170 and lifters for around $250. Doug's headers $600+, an exhaust system from Summit around $300, Oil pan (was a truck motor), oil pump, intermediate shaft, bushing, freeze plugs, and a full ignition system from 4secondsflat.com
At this point, the cost of the block drops to below the level of the TAX on the build.