2 dr sedan would be easier than 4dr. Convertible means having door gl***, and that takes a lot of planning and fab work. I have a 32 Ford coupe that lost the roof, so I made it into a convertible coupe. Lot of work to get the door tops modded and get roll up windows. Some pics in my gallery. Maybe later I will put up pics of my 32 Nash 2dr convertible sedan, maybe you can get some ideas. That car was missing the entire folding top, so i made one. That's a lot of work, too
I cut up my 33 Fordor Sedan,turned it into a Victoria Convertible Sedan build pics below, it ain't easy http://www.jalopyjournal.com/forum/showthread.php?t=570915