By far the easiest way would be to pull a head and measure the bore. A 350 will have a 4 inch bore ( as would a 327 or a 302) while a 305 will have a 3.736 in bore. The difference is very pronounced once you know what you are looking at . Now , the 400 has a 4.126 in bore, and 3 freeze plugs ( or at least a boss for the center plug even if it isn't drilled) plus a 3.75 in ( IIRC ) stroke as compared to a 3.48 in stroke for a 305 or a 350 , a 3.27 in stroke ofr a 327 or a 3 in stroke for a 302. All but the 400 use a 5.7 in rod ( the 400's is shorter 5.565 IIRC). AS for the differences in years, the earliest small blocks made from 55-56 had no oil filter boss, 57 was the first year for that, early Chevy II blocks had a relocated oil filter boss that is higher in the block ( and are front sump IIRC), in 1979 or 80 the dipstick went to the passenger's side and about the same time GM went from a 168 tooth flywheel to a 153 tooth: be sure to use the right starter! Also, transmissions designed for the 153 tooth flywheel sometimes won't work with the 168 tooth piece ( T-5's come to mind...). Also, a 400 is externally balanced and can't use the neutral baalnced flywheel of other small blocks without either rebabalcing the engine or flywheel. Finally, GM switched to the roller cam block between 86 and 87 with the introduction of the one piece seal block. However, not all engiens made in these years are roller cam; rather, they all have provisions for the cam valley spider to be installed and can use the factory roller cam valevtrain if it is installed.
1985 Cutlass Supreme Brougham (ex pizza delivery beater with 240,000 miles):
355 Chevy- 9.5:1 compression, Comp XE 262 cam, World S/R Torquer heads with 2.02's, Edelbrock Performer, 800CFM Q jet, Camaro ram air system,Edelbrock headers, Comp Magnum roller tip rockers,TH 200 4R, 3.23 rear, aluminum hood, IROC steering box, poly bushings, Eibach springs, etc. Cutlass build thread here: viewtopic.php?t=4982
Also: The Thing That Won't Die-1998 Nissan Frontier, 340,000 miles of pizza delivery.