Keep the 460 & pound on it!! Have a Blast!! A C5AE-? or C6AE-? Nascar Iron Block & head engine can run in excess of $ 20 to 25K !! The 460 can give you 700 hp for less than 10k. Drive it like it was stolen. Get used to your roadsters feel & handling. Then think about the swap! Most any quality shop can install your FE. As to a quality builder; there are a few on this site & others that arn't. If you can document the block & heads as a 60's FE sideoiler. My 1st choice would be holmonmoody.com It's hard to beat the original FE builder. Send it to them & then have your shop install. My 2nd choice would be ("Blykin's" his blog/thread tag on this & other AC sites) b2motorsportsllc.com But I am a little biased here
ENJOY THE ADDICTION!! THERE IS NO CURE!!