Secondly, FWIW, once the CRT is discharged, I like to suggest removing the neck PCB from the back of the CRT before pulling at any cables on the motherboard.
In those instances where that cable is tight, more so with the Mac SE, though I have seen the ones on the Plus type do it too.. if they've not been removed in a long time, they get stuck. People pull, lightly at first, and then more .. and more .. and suddenly it snaps off .. and the hand hits the neck PCB.
The next thing you hear is a "psssssssssssssss".
You've busted the nipple off the CRT and that's it. It's done.
Once you've got your ROM issue sorted out, if your motherboard has been upgraded to 4MB but the image on the screen has disproportional borders. The top and bottom are thicker than the sides, you can take the opportunity to adjust that, using a CRT adjustment tool set. Looking on the outside of the power supply PCB, there are some diagrams printed on the white sheet. One of them has a monitor with up/down arrow diagram on it. That is the screen height. Turning it slightly while it's on, you can see what it does. Make it "look right", to where the pixels are square, and that blinking folder isn't squished, or put a disk in and let it boot to the desktop.
There's a graphic around that shows a square box that you can use a ruler, though I just do it by looking and usually nail it.