Hi all - I have what I hope is not a stupid question

I have a Great Planes AT-6 Texan ARF that I am in the final
stages of construction - I am attempting to install a Magnum XL .46
with a Pitts muffler. I have installed the motor mount and drilled the
mounting holes 4 1/2 in from the firewall as suggested by the manual.

Unfortunatly this does not allow my pitts muffler to fit and leaves
the cylinder head extremely close to the plywood cowl mount.

I have experimented and have found that if the engine is aprox 5in
from the firewall I have an adaquate amount of room. My question is what
(if anything) will this do to flight performance or safety? I posed this question
to GP's customer service and their advice was go out and buy a slimline
sports muffler - yeah great except I have already forked over 40 clams for my existing pitts ! Will the 1/2 in really mess up anything or do I have
to go spend yet more money ?

Any of you warbird fanatics have any good tips regarding this kit - specificly
with regards to making it just a wee bit more scale ( I have already done a job
on the instrument panel - and installed a rear one too ) as well as improving a
little on the ( somewhat lame ) decals and covering job. I would also be
interested in any feedback re flight characteristics ( FYI this is my 3rd plane
but my 1st warbird and low wing aircraft ) Thanks

Christopher