Thanks for the inputs. I'm going to do exactly that. I have a couple of scraps of 1/4" plate from my welding class. I haven't tried a vertical weld, so that'll be fun. I can overlap them to simulate the joint. I only ran beads on horizontal plate, then did a 90 deg butt weld both sides. At least I got a feel for how slow you go, what it sounds like, and more importantly what to look for in the weld puddle. I'm assuming drag weld down vice pushing up? I'm sure some online videos will help.
FedEx is dropping them off Saturday so I'll prep/paint Sunday and probably weld 'em on when I'm back from some travel.
I've used both top down and bottom up methods on vertical welding, and while they both will work, they require different technique. Going up from the bottom is the most difficult, as gravity tries to pull the puddle towards the ground, so if you spend too much time in your puddle, you will loose your filler material as gravity takes it too the ground.
To start with, let's just admit this isn't welding 101, so you are not working on getting an A in welding class. You need to get the rails welded in place and have good penetration. With that said, I would recommend top down welding in a large C shaped weld with the arc of you C on the up stroke so the opening of the c is facing towards the ground. This is essentially a series of C shaped spot welds. Start with your wire pointed at the frame, pull the trigger and make a 1/2 inch C to the foot of the mount then release the trigger. Return to the frame, pull the trigger and make another C to the foot. The time it takes you to transition your wire back to the frame is enough time to allow the puddle to harden enough so it won't just run down the material. Each C weld should take about 1 1/2 seconds, so don't go too fast, as you need penetration.
I use this method when I'm welding laying on my back looking up at the weld, especially when the weld is in a tight spot with limited visibility. Your welding instructor might not like it, but this technique will yield a good looking weld with plenty of penetration. You should also be laying each successive C in the pocket of the previous C, so don't space them too far apart. The weld should look like a stack of nickels. Most new welders can't just keep the trigger on and get a decent vertical weld, but with a quick stop at the end of each C, you will be just fine.
Expect to have to grind out your first few welds, but just wait until the end to fix them. Get the rail blocked up in the right spot and tack weld all three feet onto the frame with 3 welds at 10, 2 and 6 before you start to finish weld anything. Go to Harbor Freight and buy a 4 1/2 paddle style angle grinder (item 65519) for $33 (the paddle style is the easiest to use IMHO), a package of cut off wheels (item 61195) and a couple of 60 grit sanding discs (item 69602). The cutoff wheels work great for grinding a weld out of a pocket, and the sanding disc works best on flat surfaces.
Once you finish your first foot, you will have the hang of it and the rest should be fine, so don't get too frustrated in the beginning. We expect to see some picture when all is said and done.
