I have never used a tmp. gauge on these engines so I am not sure about that part...
As to the pipe- Shortened! Sounds like less gap is the right way to go, most recommend cutting the header until the RPM's do NOT increase and stopping there- try a search on here for tuned pipe tuning or similar and grab a cup of coffee!
I rarely run my performance engines (Anything other than idle-) for very long on the ground since there is no cooling while it is sitting still- that sounds like the problem right there...
You might also try a 10x7, my brother is using the same combo you are so I will get back to you when he flies- He is runnin 9x8 apc and I also told him to try a 10x7
