How about a 944 turbo (951) engine? I have heard of some people getting some pretty big numbers (400hp+) by upgrading the turbo on the 951. Also know that by keeping at "Porsche" you will keep your conversion costs down. Make sure your ground straps are in good shape if you are having issues.

If your set on a engine swap - maybe look at : www.pelicanparts.com and www.rennlist.com and check out the 944 forum sections.

Other sites:
http://www.powerhaus.com/944t.htm
http://www.andersonmotorworksinc.com/
http://www.tdi-plc.com/Welcome_2.htm
http://www.pors-chev.com/