I am no expert when it comes to these stroker motors, but earlier this year I did quite a bit of research into where to get one myself. The largest amount of positive things that I was hearing were about MRL. I decided to have Mike over there build me a 408. I don't have it here yet but he dynod it at 520 HP and it's a pump gas motor. It doesn't cost a dime to give him a call and chat. He won't pressure you into buying from him or anything like that. If you look through various threads on FABO you'll see a lot of happy people that have dealt with him.
http://www.mrlperformance.com/ (his website)
989-845-1547 (his phone #)