I have a 54 Desoto Hemi, a 57 Chevy 283 (front motor mounts). I also have access to a 58 348 and a 53 truck flathead. Any of which make great early style hot rod motors. So which ever you choose will be great. My choice for my 30 Model A is the Nailhead.
Joe, the debate between the 348 and hemi got trumped by a perfect 56 pontiac 316 setup with a tri power and original hydromatic ******. Pics coming up soon.
I agree with Joe Didio. They are both great options. I'm a serious Dodge and DeSoto Hemi nut (I own six), but a tastefully done 348 looks outstanding. You have a tough but very cool choice to make. Good luck. trakrodstr aka charlie