2022/08/20 - 2022 Update
Well, some wrist injuries kept me away from the car for the better part of the last year or so, but I'm back at it again!
Over the past month, I've torn down the rear of the car and removed the casing for the transaxle. Mendeola was super helpful (shout out to Ian!) and replaced the porous one with a new one. I've buttoned it all back up and we seem to be good to go.
A note to anyone that might be building a GTM, the studs on the rear casing of the transaxle strip out VERY easily. All of them have been replaced with bolts/nuts by me. This would normally not pose an issue on other applications where you wouldn't need to access these bolts. But, with the GTM, this is where the mounting plate gets attached. Long term, I see no issues, just looks a little janky (see last set of pictures).
I've also stripped off the header wrap. I keep hearing more and more bad things about them and how they degrade the metal.
Eight images above - old rear-most casing of transaxle with porosity issues
Four images above - if you wanted to know what the innards of the transaxle look like, here's an idea. A high tech dust cover (read: plastic bag) was installed on this portion while it was exposed, didn't want dust/dirt getting in there more than necessary.
Four images above - the new rear casing. No porosity issues to be seen!
Four images above - new rear casing installed. Note the 4 bolts, these were studs before. The stock studs stripped out, but this works just fine.