My take away from your post is that you need to shoot for stretching the legs out on that '54 3100.
40-45 MPH isn't going to keep the carbon burnt out of combustion chambers.
Live a little, man! Your engine will thank you.
BTW, I would use the carburetor designed for the engine. Save that 261 carburetor for a 261 replacement for the 235 that you are going to blow up by following my advice above.
