I'm not sure I understand this one...if you have a FEL, use it as a counterbalance (good idea to use the FEL in the brush anyway).
I ran my 600# cutter (5' Land Pride) with no front weights on a Deere 670. Now I only had the cutter in transport height when I was on hard pavement. In the brush or crossing the lawn, I just raised the 3PH enough to keep the front of the cutter from digging in. The tail wheel was on the ground.
But if you do need front weights, 5 of the 42 lb weights should be quite sufficient for a 460 lb cutter.