42lb injectors will be just fine on that setup.
I ran 42lbs on a wilder 408 setup, I did have to go with ls9 injectors for my current setup.

 

I ran 36# injectors in my zo6 ls6 475/415 to the ground injectors cycle was in the low 90’s, probably 540ish crank HP so that chart RedXray posted I think is about right

 

The torque of a 408 is a lot of fun---- even in a really heavy truck platform.

 

A lot of factors determine how far injectors will go - fuel system type (return vs non-return), vac referenced, pressure, fuel type, and desired AFR. With said, 42's will be fine for your setup.

What is the compression ratio?

Leave the torque management in the transmission and run a cooler to keep it cool and the trans will be much happier.