Defintely get a built motor from one of the good shops, or at the very least get the short block and do theeasy stuff yourself. I wouldn't go Jasper for me performance engine needs - might be okay for some of the low-buck Cobra builds or for cruising but if you're going to do a 40 make it right with the engine as well.
Coast Performance in CA seems good, Gordon's motors are outstanding, or Panhandle has also been highly recommended. I used Coast, but it was before I'd learned much about Gordon and some of his motors. If I went again I'd give Gordon a call.
Ron