Vince if you reach it easy you can weld it with NI-ROD, just V it out preheat it with a torch and weld about 1/2" at a time, if it's just a tempary fix i'd run the block sealer.
The 400 blk 377", that i have in my Malibu has a 4" crack 3/4" below the deck on the passeneger side. It wasn't caught during the build, the blk wasn't decked. I v'ed it out and Tig welded it without pulling the heads. I'd weld about 1/2" and pean it too relieve the stress then weld another 1/2" with NI-ROD. It has over 200 passes on it and running a 16# cap. Course when i pull it apart next time ill change the blk. out. This was done as quick fix and it's been running for about 5 yrs.
Zip.