Basic problem with those cars is that they are "buggy based" design. That means they do use the suspensionparts and their link points given in the original buggy design beeing optimised for a good ride height.
So if you look at the equivilant buggy you will see the lower arms pointing something like 10 to 15 ° down. The ride height will be 40 to 50 mm.
The GT lower arms are normaly nearly horizontal or slightly upwards, still having a ride height of around 25 - 30 mm.
To get it down to something like 6 - 8 mm front and 8 - 10 mm rear is difficult to achive with the parts out of the box. You might need other springs and or mounting points to do so.
One easy way to get 5 mm is using other tires. Donīt know what the outer diameter of the OFNA one is, supose it is 106 mm like the others from same manfacturer.
As an exaple PMT (Italy) and Kyosho ones are only 96 mm, just FYI.