Our rules state "Axles must stay in originally provided slots". This leaves little wiggle room for cutting new slots. So, I cut off the rear of the car to make 5/8" from rear and CA'ed the piece to the front. I then filled the slots with wood epoxy and drilled holes using Silver Bullet Pro. I lost a small amount of body length with the band saw cut. After cutting body to final thickness, I made up the difference by adding a toothpick to front end and sand to make full length.
Some Pros here have stated this is to much work and I should just drill holes and cut new slots over the holes. I may try this next year but was concerned it is violating the rule.
I do have some concern that the epoxy did not keep the drill bit as straight as solid wood. So, I expect you will get better alignment by drilling first and then cutting the slots. Good luck.