300whp with forged pistons and rods will be fine. It will make up to 400whp on pump gas with pistons and rods.
If you want to rev that ls head your going to need valvtrain.
I made 366whp on a stock ls bottem end that I did not care about. it went 11.2@120. The head flowed it fine. well sort of fine. it was like 18psi.
My motor now at 18psi is around 500whp GSR head skunk 2 pro2 cams
There is an issue with the ls motor and anything over 310whp. You run out of RPMS in 4th @ like 118.
Of course I will tune it.