I took time last night trying to upload the files to Foundry and found out a few things.
1) The 256 files are simply too big.
2) 175 WEBPs are importable without much loss of quality compared to a 256. However, I had to set the grid size to 256 to get the tokens to fit properly on it and the grid to line up with the exported map. I'll have to hand draw the walls and lights because I couldn't get the UVTT imported.
3) Foundry's base is actually 100 pixels.
4) I never could get any of UVTTs to load that I tried (125-256pixels) except the 70s.
Armed with that new found knowledge, I'm going to be re-exporting the Mega Lab files at some point today (I work, so It will be later today). I'll include 100 pixel variants of all the files as well as 175 pixel WEBPs. I'll try exporting the 175s as UVTTs as well but I wouldn't hold my breath on them working out (I got a black box).
Sorry for the issues, all! I honestly did not realize some of the limitations that would encompass such a large map.
Cheers,
Rija