I think they should both go in! lol seriously though being a STREET rod, go with the smaller motor. but if you are building a drag car...well then bigger IS better.
I love the 37! I have built the convertable with trailer (left out the trailer) and have the tudor sedan in my to do pile. You might consider snagging the chassis out of that testors '37 with the custom front end, they also had 3 versions the sedan delivery, a coupe, and a vert. The chassis is fantastic on those, but it takes some work, and the rest of the kit is good at best.
Don't get discouraged, People are looking at it, but there just isn't that much to see right now. You've already completed the motor, and the unpolished paint looks fantastic!