Barry may want to weigh in here, but I had about 3/4" from exhaust to bellmouth on my F-86 with no issues. If you look on page 38 of the new (updated Jan 2013) manuals (http://www.kingtechturbines.com/doc/manual.pdf) you will see that they recommend 20-30 mm (.8 to 1.2 "), so you are probably okay.
Regards,
Gus