Some sb came with 8 inch hb ... others w/ 7 inch ... others w/ 6 3/4 inch. It is my understanding that many hi-po sb came w/ 8 inch. Which vettes, if any, came with 8 inch is beyond my knowledge base.
Are you running the stock timing cover? If it's painted it's probably stock. The timing tabs are in different locations for different diameter hb. 8 inch is largest hb ... so if you still have a functioning timing tab then yours probably came w/ 8 inch. I'm building up an L48 that came with 6 3/4 ... have 8 inch with older matching timing cover that I WILL USE. Unless you're an all-out racer, many (including GM) believe an 8 inch elastomer-type hb (in good condition) is ideal.
As SSVETT (steve) pointed out in a recent post, the rubber elastomer does degrade over time as exposed to heat, solvents & stress or improper remove-install technique (don't pry or beat on it) ... the outer ring can slip .... causing improper timing index ... OR WORSE ... it could slip enough for the ring to come loose in the engine bay ... KABOOM!
A new aftermarket elastomer-type hb (for crank snout w/keyway) can be had for the typical sb for under $100 (any size, internal or external).
Another concern is this. All sb (EXCEPT 400 and those using 400 or 400-type crank ... i.e. 383/385/388 strokers) are internally balanced. 400 crank is externally balanced. They all use a hb ... but the external balance hb (400) has a lot of meat "gone" from around the engine side of its outer ring. The internal balance (265/283/302/327/350) ring has none missing. Make sure you replace an internal hb w/ internal ... and vs. External balance also uses a differently balanced flexplate/flywheel than internal.
I'd keep the 8 inch ... just ensure that it's in good condition ... if not, replace w/ new 8 inch. That is, unless you're going all out racing or its originality is a concern for showing-judging. Sorry if TMI ... hadn't posted in a while ... and your question is similar to hb questions I had a while back. G'luck!