I don't know much about the vintage cars, despite owning a '68 Amazon for a few years back in the early 90's, but word is that if you want a performance engine you are better off starting out with a B20 as opposed to boring a B18. Reason being that there will not be enough "meat" left on the B18 for the next rebuild, so that block is essentially trashed for future use - or trashed, period, if your machinist goofs.

There are other problems with boring a B18 to B20 specs. Because of the difference in compression ratios, the B18-cum-B20 actually produces less power and is less tractable than the 'real thing.'

You could probably do a straight up trade of your block for a B20. If I am not mistaken (ahem), both motors use the same head. Popular B20 mods include the Isky camshaft, aftermarket exhaust header, Weber DCOE's (requires an expensive manifold) and the ever popular pertronix/perlux electronic ignition.