If you bought the Kit sans Courage Body then many of the Tamniya TC bodies will fit without modification as the original F-103GT kit was designed to be used on a TC body.
I would stay with the TRF Tamiya shock as they jhave proven the be the industry leader in on-road shock design.
If you look in the pits at Reedy their are even several drivers from other makes running Tamiya shocks on their non Tamiya chassis as they are that much better.