Originally Posted by lowhirod
I,m after low in torque. I tow a 30 hp tractor with a half ton truck
In that case you're better off leaving it as original. Unless you increase the displacement anything you do to the heads or cam will reduce bottom end torque, push the torque peak an horsepower to higher numbers but also they will be at higher RPMs than what the current LO5 peaks at. This will drive you to stiffer final gearing to restore power at cruise back to where it was with the swirl port heads.
If you really need more bottom end the best way to attack the problem is with more displacement, and the best way to do that is with a 383 stroker kit. The other ways of getting there with the engine you have is with a stiffer rear axle ratio. This will give the engine more mechanical advantage in all gears and will cause it to operate closer to its torque and power peaks in normal use.
You have to keep in mind that new trucks since the mid 1980's are very much an exercise in systems integration design. The Swirl Port head is made to work with TBI and actually does very well with a carb. The Vortec was made to work with port injection and with that set up doesn't surender much if any bottom end torque. But when that head is mated to TBI or a carb a very noticable drop in bottom end torque is seen. The overall performance gain in this situation for the Vortec happens above 2500 RPM which is significant, but unless you rev the engine up you can't get that advantage with TBI or a carb and these heads.