for 20" high, that's good
2) 3 inch euro brace with one 3 inch cross brace?
3" perimeter, and a 6" cross brace every 24", in this case, since you are at an odd length of 60", make the perimeter 4" and one 6" crossbrace and that is probably good. Actually you are probably good with 3 & 6, James might comment to be sure
3) Thinking about changing the top piece to 60.25x18.25 to account for any of the angles not being perfect 90s and routing off the extra. Too much or too little margin for error?
You will want more than that. You need to have the front, back, top and bottom panels cut with minimum 1/8" overage in the appropriate areas. This means that the left/right pieces are cut to the inner dim with no overage, then for the front and back panels, the top and bottom edges have no overage (and these match the vertical dim of the end panels precisely) and ten the left/right edges have overage for flush trimming (1/8" + 1/8" = 1/4", and you can have MORE and that is OK as you will trim it off), then the top/bottom have 1/8" overage on all sides (so 1/4" extra for flush trimming all around, or more).
3) What's best for a newbie - Stick with weld-on4 or add additional chemicals and mix? If so what exactly (you guys loose me with the acronyms) and how much do you recommend?
WO4 is pretty decent but you can easily find a small amount of Methylene Chloride (make sure it is as pure as possible) and then some Glacial Acetic Acid and mix 95% MC and 5% AA, but if that's a total PITA then just use WO4.
4) The WO40 after - Stronger? Weaker? No effect?
Don't listen to **anyone** that doesn't build tanks. I don't care if you are talking to a company rep direct at Cyro, they don't know the real world. you do not need WO40, in fact, you're more likely to have a failure if you do. James has written up a very excellent explanation of this, in fact I think he posted it recently again in this thread. Search back for posts by "Acrylics"
After the tank is completed, to really over build, I'm contemplating using WO40. I plan to build a jig so the seams sit on a 45 degree angles with the floor and pour some WO40 on them so it runs 1/4 to 3/8 up each of the sides. (it would look similar to the silicon around a bath tub) When I saw this it was mainly to repair old tanks but stated it could be done with new tanks as well to increase seam stability.
Your choice of course, but this is IMO not necessary. If you build the tank properly in the first place, and set it on a flat level stand with proper support and 2x 3/4" plywood sheets (no foam) the tank should last you a long, long time.
5) Suggestions for a stronger more idiot proof build of this size?
Read back in this thread quite a ways, ask questions, learn the pins method, and tackle a small project first. Like making a small sump or pico tank, just out of 1/4".
Learn edge prep, how to use a router table (passing the material between the bit and the fence is something that most experienced woodworkers would NEVER consider...but it is a necessity here).
Make sure your euro cutouts have 3" minimum diameter (1.5" radius) on the inner corners
When you say this
Local acrylic supply will be doing the cutting and polishing.
1) rough cut edges are not good to work with. So you need to have any edges that are going to be bonded to other edges cut with enough overage so that they can be routed to weldable quality edges. Flush trim edges don't matter. End panels must be square, as perfectly square as you possible can get, if not perfectly square, they need to be off by the same amount (i.e. double-stick-taped together and routed at the same time)
2) polishing.....I bet they are thinking of flame polishing. Don't.
