No problem! Glad for anyone to have the opportunity to trade info and chat about these trucks. I'll try to add what I can that could be helpful.
I wish I had better pictures of all this stuff! But here are a few with some thoughts that I hope you can use in addition to what solidaxel has already posted. He's right on that once that area is stuffed with hoses, there really isn't enough room for much else without some serious re-engineering. Here's how we tacked it on Marvin.
The first component was sealing up the fresh air grille. I wanted to keep the grille on the exterior of the truck instead of shaving it. But I also wanted to seal it up on the inside to keep the elements and road noise out. So here you see a cover that was fabbed and welded to the inside. The side of this cover that faces out is painted with POR15, and there is a small drain fitting on the bottom right corner of the cover (that you can't see in this picture) with a pvc tube running down to the bottom of the inner cowl area (where there is also a drain opening to the outside). Very important when sealing up any part of these trucks to consider drainage for any moisture that does happen to get inside! Don't want all your hard work to rust away before your grandkids get to drive it.
Then, a simple cover was fabbed for the inside-the-cab opening. This piece just bolts on. You can kinda see it in this picture.
Sounds like you've seen how we are routing the hoses into the cab. There are individual grommets for each hose through the sheetmetal where they enter the inner cowl area, but I don't have pictures that show that, unfortunately.
And then here's a pic showing how they all come up inside under the dash, very similar to the other two that were posted. It gets pretty crowded!
This is the best pic I have (unfortunately) that shows how we ended up "sealing" it all up. You can just see in the top center of this picture what we did. We just used that flat cover plate and bent the top out to create an opening for the hoses to come through. It isn't the most pretty, but it won't be visible once the glove box is installed, and there really isn't enough room to do a typical grommet or bulkhead connector approach. We will maybe stuff some sort of material in the opening around the hoses during final install for noise purposes, but I think moisture and outside air is pretty well taken care of already with the grille plate and the grommets on the hoses where they come into the cowl from the fender.
I hope this gives you a few ideas! Happy to answer any other questions to the best of my ability.
Also, while I'm here, just a short quick update on Marvin himself. I moved to Iowa in May, but Marvin is staying up in Minnesota until he's finished up, so I haven't been able to stop in and check up on progress lately. Same story as last time, though - body work body work body work. Getting all ready for that pretty red paint. Hood fitment was a real pain in the ass, apparently. But, as of last week, lots of block sanding happening, so there's some progress for sure. I'll definitely get some photos the next time I'm able to zip up to Minnesota!