Ok guys/gals, I just called my trans guy and here is what HE had to say about the two transmissions.
A TH400, can
NEVER be put into nuetral and the engine cut off as what happens is, now the engine is not controlling the drum in the case at 7000 rpm's (as an example engine rpm), and since the driveshaft is now spinning the drum at 2.2 times the speed of the engine that was, at say 7000 rpm's, the drum is now spinning at over 15,000 rpm's and BOOM, there goes your trans.
The powerglide is able to handle this type of shutdown as the internal parts connection are a little different so it is ok to jam your glide in nuetral, and kill the engie to do plug reading at the turnoff of the strip, whoo, got me to thinkin there for a minute :?
Just some info that may help others out later if they don't know what or how to do this that want to read plugs for tuning. Now I know the rest of the story, Paul Harvey, Good Day!!!!
Elkyman
John