Upside down in the footwell while trying to work on the panel is a horrible experience.
The experience: First you remove the seats. Your legs and butt hang out the door, you place as much cushion as you can find on the door sill to pad your back, your torso and head are down hill in the foot well, you've placed your tools exactly where you need them, because there is not enough room to turn and look for them when your are in position. Then you realize you still can't see what you went in there to work on. Now you want to get the mirror and pen light. Try and back out of the down hill position that you stuffed yourself into.
Each time you drop a tool, or a fastener, it bounces off your head first and then dissappears somewhere next to your ears. You can't move your arms around freely because the footwell is not big enough to let your elbows bend.
I'm sure you get the idea...
If someone else does your panel work all of the time, it's probably not an issue. If your panel isn't 40+ years old it isn't an issue. If your hydraulic cherry juice doesn't leak, it isn't an issue. I always seemed to find a reason to go back there to see, fix or fill. (with mechanic approval, where necessary, of course)
My M20C had the single piece windshield in the position of the original windshield.
Outside access is a beautiful thing. I suppose the look and speed of a sloped J windshield is nice also.