You might also try the FE Forum; lots of big horsepower engines, and some very serious drag racers.
http://www.network54.com/Forum/74182/
If you post the same question you'll get some suggestions. Everyone will want to help you get out of the BBC
20K might be a little tight though.
A 511 inch FE engine can weigh under 500Lbs with a bottom end that can take up to 1500 HP
Good luck whatever you decide!