I've converted the MTH Centerbeam Flat to Atlas trucks. You do need to dismantle the car, as the MTH truck screws are fitted from the top, rather than from underneath.
I use the Atlas truck with the plastic 'spacer' attatchment supplied with them as spares, with a small metal washer to help centering, as the MTH hole is a bit wider than the Atlas piece, & I also use a thin rubber 'O' ring of about 1/2" overall dia. between the truck & frame to reduce 'wobble', as tightening the Atlas screw right up still doesn't hold the truck up against the car.
Sure someone has posted on here before about it, as it's more-or-less their method I copied!!
Best thing about the Atlas trucks is the rotating bearing caps, of course. The MTH 'roller bearing' trucks don't have this feature.
There should be holes in the underframe that are Kadee-compatible, though you may need to shim the draftgear boxes down a bit to get the coupler height right.