Oiling, mostly. The 7m oiling system has known issues, that can be corrected with a bit of work. I've also heard that the 7m likes to cavitate at higher rpm's, but I believe IJ found/made a fix for that as well. Also, it's been mentioned, piston speed. It's going to be rather high above 7500 or so... if you plan on rebuilding them once in a while, no big deal, but you're pushing the thing awfully hard past a certain rpm.
This is an option I've seen for guys wanting to stay NA and high rpm.
If all you're looking for is ~300whp, I think a 1uz would be a better option all around. Not to mention, it sounds like you're using the car for auto cross, I get that, and approve.
My reasoning for going with the 1uz?
-It's significantly lighter than the 7m, even the NA 7m.
-It's shorter in length than the 7m, which will be better for weight distribution.
-It will make more torque than the 7m, somewhat eliminating the need for such super high revving.
Plus, there's the sound. Check out the Youtube videos of the 1uz with ITB's. You can thank me when you're done changing.
I'm more or less indifferent to the project, but I like your attitude. Obviously, you're going to have people try to sway you away (some with good ideas, others with just good intentions, some just being asses...), but I like how you're handling it. If nobody believes it's possible, but hasn't proven it or have damn good backup on why it won't work, I say go for it, prove us wrong. Just be prepared for even more headaches (both financially and emotionally) than you've seen in this thread...
A 6m block with a 7m head and custom pistons *is* a 7m.
