Ok The F103GT is a 190mm wide platform set on touring car wheels and tires.
The F103RM or F103 is a 200mm Wide Platform set on foam F103 tires and Rubber F103 slick.
So you keep going after the speed run of it all. so if you are going to run in a straight line or oval ?
There are way to many things to take into account for a speed run.
I will answer your questions the best I can but everyone will have input and I dought any will match others kinda like the nose on your face everyone has one but there not the same.
That body for the most part will fit but it is a road race body designed to help a car get around a track with downforce injust the right places.
So it is hard to know what it would do at speeds over 100 mph or even 60mp.
now for a speed run you only have so much room for gearing for a speed run one would want to get as close as they could to a 1 to 1 ratio and with a tamiya F103 or GT there is not that much room for gear or if there is even gears made for the tamiya to do this but I am not sure we run F1 cars and pan cars for road racing.
Touring car tire vs F103 tires - Well there are plus and minus to both and you could always run Nitro foams on the F103GT.
we could go into the tires and spend a lot of time on it so a narrow heavy tire like a touring car rubber slick should give you speed and help keep the car on the road. But the foam tire should spin faster less weight.
The body? don't cut out the front or rear wheel cutouts so wind can pass by it but you will not have much steering.
Why would you want to do a speed run with a F103?
Last you might want to post a new thread like Help with a F103 or F103GT Speed run.
I would think the Oval guy's would be the best to ask. or here
http://www.rctech.net/forum/electric...ps-tricks.html
Also most speed runs are done with caped tires and the F103 or GT can not run these but the guy's running oval in Japan (d-Drive series) have caped tires for this.
Also the parts needed to go from F103 to F103gt = Front suspension complete rear axle form one to the other. so if you have a F103 then you need to get the F103gt parts and the same if you have a F103gt then you need parts for a F103gt.
Not sure if this helps or not so lets see what else comes up here over this.