Agree with Bevar, I'd suggest 25-35mm gap from the end of the tailcone to the front of the bellmouth, and also make sure there is some way to get additional cool air drawn in to the space between inner and outer.

Depending on the install, I've seen a lot of guys open up a set of vents in the belly of the jet just ahead of the beginning of the outer tube, you can make them with the NACA vent opening shape and it even looks cool :-) I did that on my Yellow F-15 and F-18 setups. Perhaps not needed in your setup since you don't have a bypass...

Dave