I can only speculate, but GM did the same thing on the Olds 307 too...conveniently, after the 305 and 307 were both chosen to replace the respective divisions 350's. As the story goes, the 305 was supposed to mean the demise of the SBC 350. I find it odd GM made that miscalculation.
From testing I'd seen from folks like Vizard, the large CFM carbs help torque on the smaller motors. This flies in the face of "conventional" carb wisdom, but if you've been around long enough, you'll recall that the way that carb sizing was previously recommended, was even more basic than it is now.
I would speculate that since the difference is on the primary side, the additional CFM helped before the secondaries came into play. Probably let GM use the secondaries less often, which likely tended to improve economy, drive down emissions, and "feel" better to the driver since the tuning of the primaries is more precise.
Side note, the HO 305 in the Monte SS of the same vintages got the 750 variant. I don't know much about the b-bodies, but I bet the heavier cars with worse gearing probably got 800's. As the Old's 442 got the 800 as well, but had 3.42s vs. the SS' 3.73, I feel that also indicates GMs rationale.
AFAIK the 800 was the norm on 305 trucks. I didnt pull a ton of them off the trucks back in the day, but if I knew the truck was a 305, invariably I found an 800. I dont know if the bad gearing/800CFM combination was across the board on the truck 305. I cant recall if the 305 trucks could have been had with 3.73's. Would be interesting to know...OP might be able to answer that depending on the gears in this particular rig, if OEM.
