There is very little info out there on 400 HO cars as they are rare and don't get the attention the RA motors get. Little sales data to compare but I have seen a few over the last 7 years prices north of $60k and less than $75k.
Green bananas ripen, LS motors don't.
I'm curious to see if there are documented sales (i.e. auction) in that range. I'm not saying it didn't happen, heck even this happened: https://www.mecum.com/lots/SC0517-282571/1968-pontiac-firebird-convertible/ Yes, 75k for regular 400 convertible. Sure, original engine, triple black, etc. But hey, I should stop knocking it and hope I can get anywhere near that for mine(non-original engine/trans).
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto