I don't share the apocalyptic views of the 403 like many others do. Windowed mains does not mean weak mains. Mondello used to run 500+ hp in a 403 without a main girdle, so street/strip power is just fine if you ask me. He also did it with the block filled with cement up to about 1" from the deck, so overheating isn't a big issue according to Mondello.
But... most of the 403s I've built are street-only so my experience with really hi-po olds is limited. I had a 403 in an 87 Caddy making about 390-400 hp on a 30-overbore and I never had a single overheating issue idling in traffic in the Florida summer with the AC blasting... and that was using the stock 307 radiator.
I personally like the 403 and have found no particular weakness with them for street and street/strip use.