There is a ton of great maple syrup information on a couple forums that you may want to visit.
Sugarbush.info and
Mapletrader.com. You can spend months looking through the designs other folks have come up with.
How many trees do you plan to tap? And/or how much syrup do you want to make? That would help us guide you better.
So your pan would be half of an oil tank? I'd be somewhat worried about that. I'm hoping it would be a clean/unused one. But doesn't it still have paint on it? If you can get all the paint off before you cook your food in it, that would be preferable.
The first picture you have of the cinder block arch (an "arch" is what they call the part that holds the fire and the pan) is a good way to go for outside use. I've used a block arch for about 6 years now. I've never had a cinder block explode, even when they're getting wet so I suspect that's a rural myth. They do break down due to the heat. But I've usually gotten a full syrup season out of a set-up when it isn't protected. If you get some sheet metal and/or fire bricks and put them on the inside between the heat and the blocks, they last a very long time. I had 4 years on my last arch without cracking any blocks by including that protection.
The key to fast boiling is lots of surface area. A 4'x5' pan would give you a lot of surface area but you want fire under the whole thing. And I find that you want the hardest/hottest fire you can get. Lots of heat goes up the chimney but a lot goes into the pan. I aim for wood to be about wrist sized and fill the firebox every 5-7 minutes. A big oak log on the fire really slows things down.
With a round bottom pan it would be kinda hard to keep the fire contained under it. And as the sap evaporates and the level drops, the fire really heats up the part of the pan that isn't covered by sap and it starts to burn. By the end when you are down to syrup, your pan would be very shallow and probably scorching a fair bit. In the olden days they boiled syrup in huge kettles over open fires. You could do that and save all the money of building an arch. You'd want to be able to kill the fire or swing the pan off the heat as the syrup finishes.
Many folks use oil tanks as the arch and cut out the top to put steam pans in. So using the pans from your first picture to hold the sap suspended in the metal oil barrel cooker. They make door kits for barrel stoves that can fit on the oil drum. The downside is that you can normally fit only a couple pans in them which gives you a relatively low boil rate.