why is my iroc z start hard when warm

Start hard as in it turns over fine but is hard to fire or the starter is having issues rotating the motor? For the first issue you should have somes codes flashing. If not you can check some simple things like the fuel system. Check the idle air POS, clean it if it's dirty. Or a bad O2 sensor. I have not had a TPI so hopefully someone else can help you with it's quirks. (I know the POS TBI system pretty good.) For the second point check the clearance between the starter gear and the flexplate. You should have gear mesh but they should never bind. Heat of the warm engine may expand components and bind the gears. Is your new starter that exact same as the old one, if not or if it is binding you may need to install shims between the starter and the block..

you have modified your car, so who knows? that's why people just like to put in the 350 crate motors with carbs- no problemo