I won't go into great detail here but....
First off a big block IMO is too huge for something like that. Sure a lot of people do it but for a small 30's car? Disk brakes wouldn't be needed either but are nice to have. I also do not think the so called "rat rod" guys approve of having disk brakes on their ride.
Speedway Motor's sells the whole frame kit. www.speedwaymotors.com
For the body, for a Model A it will be pretty expensive. Those are hard to find and are a lot of money. So that will be the hard thing to get unless you go with a glass body.
Maybe someone else will chime in here and help some more. Look around some more for better understanding and idea's on building a Model A. Great place for pics.... www.carnut.com
Good luck!