bigbeef, welcome aboard. Yes this is addictive. Your only chance is to get out before you get deeper into it, BUT we all heard that and didn't listen.
It sounds like you have pretty detailed plans. That 2x5 flat pan should get 10 maybe 12 gph on a rip roaring fire, adding some drop tubes will help even more. Just be sure the "old copper" doesn't have any lead solder in or on it. If it has old solder it maqy well contain lead. In that case, cut off any old solder joints and flush the inside well. If any solder got inside it's not likely to stick because the pipe in away from a joint would not have been hot enough. I like the idea of 5/16 drop into 3/16 lateral lines, just if you can have 10 or even better 20' of drop on the 3/16 before you go into a mainline. The 3/16 will give you the natural vacuum, the mainline (or even if you used 5/16) you get no vacuum.
One thing with 3/16, in year 1 it is great, but at the end of the season you must clean the lines (if you use calcium chloride it leaves nothing for squirrels to smell, sodium chloride leave a salt which squirrels love). To clean it, plug the bottom and fill the 3/16, let it set a minimum of 30 minutes then drain it. Be sure to fill every drop line. Then repeat in mid fall - late fall before things freeze. Flush with potable water and change all fittings. Little scales of sap residue can flake off and plug the small holes in the 3/16 tubing fittings.
Your plan sounds ambitious for sure. With the gravity you will get using 3/16 on clean lines you can get 1-2 gal a day on good flow days. You may want to figure 6-8 taps per hour boil time if you get 10-12 gph on that 2x5.
Don't be like I was my first year. I had a 2x3 flat pan, which evaporated 5-6 gph. I started with 2 taps if I recall, the sap ran 1 day and I said, "I can handle more", so I added taps. Each time thereafter I'd get it boiled ok and I added more taps. In the end, I had 70 taps if I recall, then the good sap flows started. I had too many taps to keep up with. Between my wife and myself we boiled 24 hrs for days on end, taking turns. That was also not with any vacuum, it was just 5/16 drops into 5 gal jugs. Your set up will likely boil slightly more than 2x per hr. what my little 2x3 did. Be careful not to take the fun out of it.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.