Yes, they are both annuals. I will guess that the 'yellow' looked like low-hanging fruit to MPC when they could nearly match the Boss "School Bus" Yellow.
Likewise, I will hazard a guess that the inner 'core' of the mold tool was common, but the slides for the roof, f/r and sides were specific to the year. The '70s have much less frou-frou on the outside; I haven't looked at mine lately, but IIRC, MPC added 'Mach 1' engraving to the lower panel.
1969 body got around the block a few times. in the "Going Thing" '69 F/C, the Ohio George "Mr. Gasket" AA/GS, finally in the Dick Trickle #99 kit, one of the most accurate short trackers ever tooled. In the early 80's, MPC brought it all back home, cleaned up the body and re-released the 1969 Mach 1.
B/C MPC never updated the chassis from the dimensionally smaller 1966, the later kits have always suffered from a narrow track and a too-small shelf look. Still a good slump buster, don't overthink and have a good time.