Turbo 250
Crossflow 250's are very under-rated motors. You have heaps of options that (as usual) are dictated by budget. A mild 250 (MILD!! cam, ported exhaust side on head and big valve springs) should make around 200hp. A warmer 250 (Mildish cam, pocket ported head, springs, recurved dizzy and revised intake) could see 250hp+.
Turbos are another thing altogether, LPG is the easiest option or if youve got the cash - aftermarket fuel injection (MoTeC, Autronic). You can keep the motor standard and let the turbo do all the work which will still make a lot of power (around 300hp+), and torque would be sky high(around 600nm). A bit of work to the motor and 400hp isnt unachievable. Strength wise 250's can make a LOT of power on a standard bottom end, some guys even push over 300hp. Theres a guy making 730hp in his 250 cortina using ACL pistons, shotpeened rods and a standard crank, and that things seen regular strip action now for 4 years.
Ive seen a few supercharged 250's the results were mixed, in my opinion not as good as the turbos.
Btw, Im building a TE cortina turbo, worked 250, warm cam, Autronic ECU, 3" mandrel bent exhaust, custom fuel system (big pump and surge tank), kitted C4 and 3.25 LSD in the rear. Should be good for around 270-280kw at the wheels.
Check out
http://www.fordsix.com/cgi-bin/forum/YaBB.pl?board=aussie
and the old site
http://pub41.ezboard.com/bfordsixcylinderperformanceforum
Heaps of Info there
Good Luck