Tmpgenc gop & ac3 questions

[spiritgumm]

I need to change the specs (resolution & audio) of some short video clips so I can edit them into a movie. These are all ntsc vobs. I just got Tmpgenc Plus 2.5, and have 2 questions. I installed the ac3acm decoder but it's not being utilized by the program. I know I can demux and change the audio with Belight but it would save me a step if I could get Tmpg to do it.

I'll be editing in Mpeg2shnitt and reauthoring in TDA. My other question is what's the best GOP setting. I've seen various recommendations. Most video I run thru TDA usually has 15 GOP per thumbnail, including my main movie, but the standard GOP setting in Tmpgenc produces 16-18. I was informed in another thread that this doesn't matter, but I'd like to be consistent (and there must be a reason why I see most video at 15 GOP). What's the best settings in the GOP window?
I've read I=1, B=4 P=2 Output Interval=1, Output bitstream unchecked.
Also read I=1 B=7 P=1 Output Interval=0, Output bitstream checked --- but from my trial and error, the Output Interval would need to be =1.
There's also the preset "I P Picture only " which is I=1 P=14 B=0.

Any definitive ideas about these two questions?

[redwudz]

I can't answer the GOP question, but my version of TMPGEnc Plus doesn't do AC3. I process the audio separately to AC3 and add it back with the video when authoring. I just use TMPGEnc for video encoding. I use ffmpeggui to convert my audio to 2 channel AC3. Aften reportedly does this a little faster. Or there is a AC3 plugin for TMPGEnc.

[spiritgumm]

As I wrote, I know how to demux/convert the ac3 audio. I'm looking for an ac3 decoder so Tmpgenc Plus can deal with it.
Not sure what you mean by the Tmpgenc AC3 plugin. There's one for TDA but I don't think it can be linked to Tmpgenc Plus directly, which is what I am asking about.
From what I've read, the only Plus-compatible pseudo-plugin I've seen is the Ac3acm decoder. I installed it but it's not working for me. Maybe I have to uninstall my other ac3 filters?