IMHO, this doesn't really sound like a bad idea at all; however, what are you planning on using for fuel control? I'm not sure if the quench of the p90 head will be ruined with the L26 flattops, but as you read above...it's been done and it was sucessful (I'm assuming with no detonation lol). A stock l28et can take upwards of 300whp on pump gas, so with the stock T3 turbo, p90 head and the l26 block some race fuel, I think your goal, 300hp(not whp) should be attainable, but not without an intercooler and race fuel(just to be safe). If you haven't looked into megasquirt for fuel control, you might want to check it out...but be prepared to learn A LOT. Good luck and keep us updated if you decide to actually do this, this should be interesting.