I did the upgrade of the Mantua gon years ago, based on an article in RMC. The results are OK, but the car still sits too high, despite filing down the bolster and carving curved grooves in the floor to provide clearance between the flanges and the floor. If I were to do it again, I think I would cut curved slots in the floor to gain additional flange clearance and further lower the carbody to where it should be.
Note that whatever carbody height you are able to achieve, it will still take some carving on the ends and floor to create a space for coupler boxes and some fussing to mount couplers at the correct height.
East Norriton, PA