My first guess is your secondary air valve needs to be adjusted.
If you don't know what I'm talking about get one of the books or check your manual.
A delicate screw winds a spring -locked in place on the bottom of the shaft with a set screw. You need a tiny screwdriver and a tiny hex wrench. Its adjusted like setting your timing on the road.
Set it too loose and floor it, should stumble when they open. Tighten a bit- 1/8th a turn is max and repeat. Of course this under load not in the garage.
My further guess is your light car isn't opening the air valve early enough compared to the heavier caddy. In other words your running a tiny 2 barrel, just the primaries.

The seconadary metering rods are vastly different for the different cars. I think the caddies are blunt center punchs compared to the vettes thin stepped needles. You can easily swap these with no heart ache from your original carb. Single screw on the hanger, pull the hanger straight up. Swap needles or better, install the original hanger with needles since these can be different too. Unscrew them both and just swap.

Been a while but I think I kept the blunt -all out almost immediately- needles for the thin ones.
Again been awhile but pretty sure a q-jet relies mostly on changing primary needles on just a few jets, you normaly don't touch the jets

Your old electronic carb bounced these primary needles in the jets basicaly randomly with little control- a solnoid was powered on and off. It worked on an average idea.The mechanical carb uses a spring in this well to do this without bouncing them. Vacuum against the spring controls the needle level and isn't adjustable. This is why the electric ones suck. That and they cost like EFI costs to replace.
So another option is to keep the vettes base and install the mid and top of the caddies carb in. Just swap the vettes primay needles... but of course this involves a rebuild. This does get you the "right" carb but a mechanical not electronic one.
You still need to adjust the air valve even if you swap this part too.

My swap was from a 472 to a 427 so the carb didn't seem to notice. I've since changed to the 850 cfm california? q-jet with the adjustable power valve.

 

There's nothing at all wrong with running a Caddy carb on a Vette. Set it up to its stock specs for the Caddy, and you'll be running pretty darned good - it will give you a very good baseline to do some fine tuning. The Caddy setup will be slightly on the lean side, so you may find a need to fatten up both the primary and secondary sides of the carb for best performance.

The problem with any used Q-Jet carb is that most of them are set up poorly and just plain wrong - too many people have screwed around with these old carbs. Any lack of performance with the carb is not due to the fact that it's off of a Caddy - it's because it needs a good setup. I've run Caddy carbs, Buick carbs, Pontiac carbs, and a slew of aftermarket carbs on my Vette, as well as running Vette carbs on my big Pontiacs. Big block, small block makes no difference - a carb doesn't know what size engine it's on: it just meters fuel based on mass airflow through the venturi, and at idle and part throttle, any V8 engine is going to pull about the same amount of air and fuel. That carb can easily be used on your engine with excellent perfrormance.

Lars