Crate 225?
I've been researching this same subject recently. there are a couple out there, this one I actually know because I grew up in the area and I've seen several of their builds (this was 10+ yrs ago, but looks like they are still around, so guessing they are still good)
http://rebuiltautoengines.com/plymouth-L6_3.7_225-engines.html
the other one I've found is CME, they are also long time racers/builders and looks like they'll do a full build including dyno if you want to go full out
http://www.cmengines.com/