3.6 mods?
 

I have a good running 3.6. Has the typical nose bearing leak. Otherwise no complaints.

I would like to go to ITBs and bump displacement and maintain decent MPG but don't want to reinvent the wheel.

I have a 915 with WEVO upgrades after I destroyed second and third gear 10 years ago.

While money is a concern, I'm not adverse to spending it.

Is 3.8 my max w/o splitting the case? Are the PMO throttle bodies the schizzle?

PMO throttle bodies are not the "shizzle". They are beautiful, but other options yield more power at the same (ish) price.

Slip fit cylinders dictate the max displacement if you are not opening the case.

Regardless of what ITB you go with, the choice of aftermarket ECU and the tuning done to it will dictate the MPG, as well as power. You can have great MPG with an ITB set up if tuned properly. But when you have a motor with amazing throttle response and great power, the MPG goes down the drain because we tend to "put our foot into it" to enjoy the new engine characteristics.

Cheers

Step 1# (where I'm @ also btw) Find the resource that is going to tune & support your ECU conversion effort on an ongoing basis.
Will the tune come from a remote desktop session @ a Dyno?
Will the person be @ the Dyno doing the tune?
Where's a Dyno to use as needed?