First, let's be clear. Your stock car makes 240HP at the crank. With a 20% drive train loss that's about 192WHP. To make 300WHP you need to make 375HP at the crank.

In short, those mods will not give you 108WHP or 135HP at the crank.

To answer your other question, colder plugs would be a good idea when dropping to the 3.4" pulley.