If you wish to use sedan type wheels and tires there are a few additional things you will need.
The front Bumper for the F-103 GT fit against the chassis plate which was different than the F-103. The F-103GT uses a TA05 front bumper and boddy posts. So you will need a bumper set
and Chassis plate.
The front steering knuckles
are different and carry different axles
as the axle turns on the knuckle and not the wheel. This also required a special link type front end
so the suspension actually would work.
There are special rear body posts and supports
to hold the rear of the body.
You will also need new rear hubs
arrangement from the F-103
I know this because I converted an F-103GT to an F-103 and I have all the parts sitting here.
So if you want to use sedan wheels and hubs, let me know because I still have all the parts available from my car, some parts I have new in package that I purchased as spares.
You could just run F-1 style wheels and tires and only get the rear body mounts. You could use an F-103LM (LeMans) front bumber on an F-103 chassis, or run body posts in the extra holes on the F-103 front end, but if you hit something this could break the front suspension.
If you want details check the Tamiya America web site for the parts sheet for the F-103GT. You should be able to see all the parts I spoke of.