Where there's a will, there's a way. If you really want a FFR with a big block, you can do it. I made a friend at least years' Fling, who had a FFR with a 588ci big block,
and it had a blower on top of that!!! . 1000+ hp.
So it can be done.
But as has already been said here, you should do research on all kits. I am a true and loyal FFR owner, and I love my 351W powered car. I am very happy with the car (I did not build it) and absolutely love the customer base. But there are lots of makes out there. Find the one that will suit your needs and budget best.
Steve