I believe you can buy the SG unit new for $300, which is what you're buying a 2:76 for, so $200 is not a bad offer! Of course , since he's at $500, he probably won't accept!
And, for the record, I've found bunches of 2:76 and 2:94 clutch style SG units in the big C's of the mid 60's, a much overlooked place to score a clutch style SG rear end!!!