I picked up a Rochester carb to rebuild as a spare for my '40 sedan. My engine is a '54 235. I have a manual choke on the car. The carb I found has a tag still on it and the number is 7002050 which is for a '53 or earlier 216 truck. The holes in the base have already been filed out to fit the wider 235 bolt pattern. My question is how well will my 235 run with the 216 carb? My car runs great as it is but has an annoying whistle at low RPMs. I have replaced the intake manifold gasket and checked all connections for vacuum leaks. Thanks.

I wouldnt use it. The factory carbs are usually on the small side as it is.

For the whistling: I had that problem with mine and found it was the carb to intake gasket. Try looking for a thicker one or get two thin gaskets.

I bet if you run the car for a few minutes then take the carb off you'll see gas all through the gasket.

Can you measure the venturi throat with a pair of dividers? Also the booster diameter? If the area across the booster is the same as a 235 carb I would think it may perform comparable to your carb.

Could it be whistling thru a worn throttle shaft?

From the data I have those 2 carbs are not very different in size.
The 216 carb should have
Throttle = 1-1/2" diam
Venturi = 1-7/32" diam
About 122cfm at 1.5" hg

The 235 carb should have
Throttle = 1-9/16" diam
Venturi = 1-9/32" diam
About 136cfm at 1.5" Hg

I don't know what main jets are in each carb but that would be another way to compare if you know that.

I've run everthing from the smallest "B" carb at about 90cfm to the biggest "B" truck carb at 185cfm on my 250 six, of course the big carb runs better up high but they all run just fine as spares, I suspect it wont care about that little difference in size. Should make a good spare.