Warped chassis on Monogram stockers

Has anyone here corrected warped chassis on Monogram stockers? It seems that most of mine have warps in them. Although the majority of my cars are 'shelf queens' I am building a Hurtubise Plymouth for racing and I would like to remove the very noticeable warp (this problem, like problems with many brands, seems to be getting worse recently - my best cars in this respect are the early Galaxie models).
The problem with it is that the front and rear magnets are at different heights. I am thinking that I will have to make an iron plate that includes this 'step' (and straighten any chassis before removing the rear magnet box), and then proceed as per the usual procedure described on the Slot.It site. Has anyone tried straightening these, and have any experience to share regarding techniques and possible pitfalls? Thanks.
P.S. I have already made a few mods to this chassis, but nothing is fitted yet. So far I have opened up the rear axle mounts to allow the bushings to be mounted higher (using the Dremel cutter from the SCC bushing alignment kit) and opened the motor mounts up quite a bit to allow the motor to move forward slightly and a 18mm spur and 7.5mm pinion combination to be fitted, as well as grinding the edges of the chassis to give more clearance from the body shell.
The problem with it is that the front and rear magnets are at different heights. I am thinking that I will have to make an iron plate that includes this 'step' (and straighten any chassis before removing the rear magnet box), and then proceed as per the usual procedure described on the Slot.It site. Has anyone tried straightening these, and have any experience to share regarding techniques and possible pitfalls? Thanks.
P.S. I have already made a few mods to this chassis, but nothing is fitted yet. So far I have opened up the rear axle mounts to allow the bushings to be mounted higher (using the Dremel cutter from the SCC bushing alignment kit) and opened the motor mounts up quite a bit to allow the motor to move forward slightly and a 18mm spur and 7.5mm pinion combination to be fitted, as well as grinding the edges of the chassis to give more clearance from the body shell.