I'm not sure why ya'll think an SM420 or SM465 can't handle a measely 300 ft-lbs of torque from a built I6, they come behind big blocks, Jimmy V6s and and detroit diesels. You ain't goin' nowhere quickly with one, but you can tow a house around.
I think you'll be munching second gear synchros and overdrive on the T5 weekly unless you are careful. You could improve the longevity with some mustang or merkur gears.
As for I6s producing good power, look at the jeep 4.0 with port injection, easy 200 hp with an almost 20 year old design.
If you go to the clifford website, you can download their R&D build sheets in pdf format. They will do the head for you to their "secret" specs (big bucks), but I think it is probably just a good porting. They have specs to turn an OHC pontiac 250 sprint motor into a 302 that will bolt in where your chevy 250 went.
