www.ffcobra.com There's a forum specifically for Chevy powered FFR Roadsters.
Really only two big issues, and one small issue.
1. Exhaust. The original design is to use stock type shorty headers connected to the side pipes by a "J Pipe". Simply a piece of tubing shaped liek a J.
For a SBC you can use off the shelf shorty headers and then fab your own connecting pipe.
For the SBF FFR sells long tube headers as an option - known as 4x4 headers. That makes the best power. There no long tube headers available off the shelf for the SBC that I know of. You'll have to fab your own, or pay someone to do it for you.
2. Engine mounts: you'll have to cut off the Ford mounts and weld in your own. I can't imagine this would be too difficult, considering how many rod kits are available for SBC swap into just about anything that ever had any kind of engine.
3. Trans Mount: This is a small issue. You'll need to mod the FFR mount just a little bit.
Other than that, the FFR doesn't really care what engine's in there.
It's your car, do whatever you want.