74 Slant 6 Bogs & Stalls Under Load / 30 mph

Maybe its me, but it sounds like you may have a vacuum leak. Easy to check, start car at idle and spray carb cleaner around all vacuum hoses and manifold. If the engine revs up where you spray a spot, that is your leak (check near and under carb). I am a little surprised this was not mentioned or tried before you started replacing parts.

Good luck,
RGAZ