I will take a look a the Demon 625; is there something about them that make them better? I picked up a muncie m-22 4 speed instead of the auto trans and a winters 10" champ 4:10 ratio. It's going it to a 31 Ford roadster. I thinking it should be fun to drive.
When I was in the planning stages of my build I wanted a 4 speed too. I bought the Muncie and then a buddy of mine let me drive his 31 Ford 2 door sedan. (327 and a 4 speed)
Let's just say it wasn't what I imagined it would be. I was expecting a lot of fun but it wasn't. My feet were too big and the petals were small and very close together.
The Model A has very little room for 3 petals. You end up having to use very small spoon petals and they have to be very close to each other. So close in fact that I could easily push in the brake and clutch at the same time or the gas and brake in at the same time. It was frustrating and a pain in the you know what. Not the "fun" I was expecting. I had to turn my foot sideways so I could hit just one petal at a time. It was very uncomfortable and not any fun. I went with the TH350 auto after that and sold the Muncie. I wish everyday that I had a 4 speed in the hotrod but it just wasn't a safe and fun option.
I don't know what your doing for a steering column but keep that in mind too. If your going down through the floor you will only have enough room for two foot pedals. Speedway sells a thingamajig for steering that you can put on your firewall to free up some space for 3 pedals but it's $500!!
You could go out the side of your hot rod through the cowl. (find yourself a old sprint car or a old Dodge, either of those steer boxes will work and won't cost you $500)
That would leave you more room for petals too. They still have to be close together and small but it can be done.
I'd start with this block 
