An Interesting Albeit Erroneous Solution:
When I see something like this it saddens me that the most obvious solution was not attempted and instead a much more involved and time wasting and ultimately unsatisfactory resolution was decided on. I offer the following not to belittle the would be modeler but to demonstrate where he could have saved himself a lot of grief and and now been in the possession of a fine resin OOP model instead of a fractional remunerative of the original cost.
Note the outrageous effort to make the parts fit by adding shims to the cutouts.
Note also no attempt was made to trim the rear of the track mud guards the casting fill tubes are still in evidence. The sponsons are protruding a half MM above the surrounding surface and the modeler is attempting to fill the gaps with white styrene and greenstuff.
On the flip side we have a pristine Shadowsword completely untrimmed from the Forge World Skunk Works and the novice builder tried to eliminate the casting flaws by building up the model where the most obvious remedy would be to trim off the casting vents.
Using the chisel and a small persuader gently tap the seam to sever the adherent without further damaging the resin. It is not advisable to pry with the chisel or you may shatter the resin Just tap the blade into the seam working along the perimeter a bit at a time so the whole piece comes free by small degrees without putting too much stress on the material as would be the case were you to free each portion entirely before shifting the chisel.
Here we find the cause of all this unfortunate modeler's dilemma:
The last three models I have found has these large protuberances on the rear edge of the hull and not one of the modelers attempted to dress them down to a uniform height flush with the interior sloped incline.
Please note that resin cuts as readily as cheese with the proper tools; in this case a standard Xacto 1/2 inch chisel and a hefty Utility knife with a new blade. and a small hammer.
There is never a need to build shims and mountains of green stuff when all that is needed is a bit of judicious trimming.