a B for under $600? NFW, Unattainable at that price around here....heck a 361 gets that much (short block only.... $300). Only 383 on craigs in SoCal is a 67 single snorkle $1500 pan to air cleaner, and it has center dump logs so its no Roadrunner motor.
340 Mopar Engine NEW!!! - $5,000 (Hesperia)
Mopar 360 Engine - $500 (Lake Forest)
360 MOPAR motor, broken down - $225
1973 dodge cylinder heads 340/360 cu in rebuilt - $650
not much more locally. I rolled on a 68 383 for $600 4 years ago and the guy blew me off with higher offers.