It sure is weird. I cant find anything on 8784234....everything indicates 979 like you say. She was built early in 68 though...i have the original invoice and build sheets... April-May 1968. There are casting numbers on the block that have julians that if I read them right seem to agree with the purchase date. I'm puzzled by the 878 though....
'68 400HO Coupe, 4 spd, 259 interior, Windward Blue. My other car's a Johnson 15 outboard on a '61 Starcraft rowboat... Just sayin'.