did a bbc with 13.1 on alky with a procharger and a c and s aerosol. If I were you I would build the 421 with lower compression and set it up on gas. Alky can work and work well but it is a pain in the ass to tune and a aerosol carb is 1400 with the bowls you will need. The bowls need see thru ends and they are 60 each to replace. They are made of plastic. The fuel pump must be a belt drive its the only ones that will keep up. Thye regulator must have a bleed. The wait for mine was 6 weeks and had to fight with them to get it shipped on time. Gas would be what I would do unless you just have more money than you know what to do with. If thats the case I need a power glide. Good luck .