Thanks for the comments
No the body changes arnt heavier or have any impacts on the turning or forward/backward movements as i widened and raised the arches.
Look too big of an arch when you look up close to them, but for what i was aiming for, they look ok.
As you can see on the 370z and the bmw, the wheels stick out a bit from the body
nothing i could do about that but the alfa sits quite nicely. I did try to gently warm the arches and pull them out a bit but it was just too far a reach.
As to what bodys to use... only thing i did was complete trial and error. didnt even measure up just guessed based on cars in life having longer chassis (after realising that i had no chance at all with the mini shells).
In an ideal world maybe if you went to a local slot car shop with your realfx chassis, and asked nicley... they may let you eye up the chassis against the side of other slot cars so you could get the closet fitting ones.
Then a little dremil cutting/grinding should let you fit the bodys onto the realfx car without having to adjust the car its self.
Just a word of warning though, the servo cover in the front of the chassis is quite bulky and does hit the bonnet of the shells... this is why i had to cut open the 370z bonnet and mold a new one afterwards, but the alfa and bmw JUST fitted within.
Have a play round and get some cheap shells from ebay
If i do any more i will post pics