there'll be numbers, unless someone went to a lot of bother to grind'em off! look on the sides of the block and on the bellhousing flange at the back, drivers side - you won't figure out what it is otherwise
here's some
general stuff about 350 family - and here's a place that'll pinpoint
casting number locations [scroll down to links, "CHEVY V-8 CASTING NUMBER LOCATIONS" - also a page of the numbers there
Bill