SOLVED: I managed to get the system to boot on 7.5.3 but it would fail with a vague memory error after trying to launch Finder. I started thinking about why 7.0.1 worked and 7.5.x would not. I'm not sure if it is strictly "correct" but a hunch from this line of thinking did lead me to an answer. My thought was this - 7.5.x probably uses more RAM during the boot process - so maybe the RAM the OS is trying to access is bad? Possibly a bad SIMM? I opened up the Plus and found 4 3 chip 1MB SIMMs. I've never seen those used in a Mac before, so I wasn't sure if they were bad or if it is some sort of compatibility issue. I replaced the SIMMs with 8 chip 1MB SIMMs from an out of commission SE and the system boots 7.5.x just fine. I've rebooted a few times with 100% success since the swap.
I'm trying to get a Mac Plus up and running and I've got 7.5.5 installed on an image in my BMW FloppyEmu. It boots to the MacOS screen, gets about 10% into the loading bar and either hangs or starts opening infinite blank windows.
What is the max OS for the FloppyEmu? I assumed it would be the max OS of whatever PC I was using it on, but I noticed the BMOW site ships specifically 7.5.3 on their SD cards.