1949 Plymouth Special Deluxe

Original carb would have been a one barrel. A slant six carb will fit and work well, but will sit lower than the OEM flathead six carb. I ran one on my 49 Dodge for years, eventually replaced it with dual two barrel Holley/Webers (or something similar, can't remember for sure). I put the dual carbs on for looks, but they noticeably added power, too.

Can't remember what year slant six carb I used, but I believe it was an early one. Factory 49 Dodge linkage hooked up to it with no modification. Later slant six carbs I think have a different linkage and might be harder to hook up.

Or get a professional to rebuild an original carb. The restoration shop your friend took his car to obviously has no clue about early mopar carburetors.