Where are you located? I'm in Brisbane.

The V6 and V8 transmissions have different gear ratios, different clutch packs and a different sized tail shaft yoke. The SRT6 transmission is the same as a Mercedes NA V8 transmission from the same era. If you've already got a spare NA transmission, then I'd use it, it will work fine. You'd just need to swap the yoke and change the gear ratios in the TCU. I'd also use the torque converter from the NA. The NA torque converter is smaller in diameter but has the same internals (ie. lock-up clutch), so it will act like it's got a higher stall speed (some companies sell this same converter as a "special" high stall converter for the NA and supercharged V8s). Send me a PM, I can make you up a tuned TCU with all the correct settings.

If your going to change the transmission, then another option to consider is to fit the 6 speed manual. I can help you with that as well. The 6 speed will have no problems with a stock (to mild) SRT6 and the NA auto transmission will definitely not have any issue. I believe the main reason that the 6 speed manual wasn't available on the SRT6 was that it would have been too hard to meet emission standards. The M112K engine did not have EGR and a manual trans would have just made things worse.