I would not consider running a later 800cfm carb on a car that is otherwise completely number matching;
Doing this will mean that you cannot run the original aircleaner, unless you find a later divorced choke chevrolet truck 800cfm unit - and I don't believe those were ever made with the front faceing fuel filter housing.
(The few post-1975 Chevrolet front-fuel-filter quadrajets I have seen were all big block 750 cfm units.)
I'd stick with the 1968 Pontiac carb that's there, or get the correct application unit.
As to replacement throttle shaft bushings;
Just a word from what I have seen;
These installs can go wrong and end up making matters much worse.
I have a 1973 400/4spd carb which the primary throttle shaft had seized in the replacement bushing - when I un-seized it, the bushing came out with it.
I would honestly play throttle shaft bushings like well plugs - if it's fine, leave it alone.
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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.
1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
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