Ben one question have you looked in grainger for splined Lovejoy coupler halves? You can purchase them pre-splined whiich would be much stronger then welding a splined coupler halve to the splined plate you have. Those couplers are cast welding to cast is tricky and in most cases you'll get hair line cracks along the weld. That plate appears to not be cast welding dissimilar metals can be a major PITA.
I'd go on Grainger.com and look for a splined coupler to fit your trans and motor shafts. You probably could have the guys at the grainger store measure up your shafts and find you the correct one. Those electric motors have alot of torque I wouldn't want to see you go through all that work and have the welds break or the coupler fracture and the ears break off from welding. Also those Lovejoy couplers are a more solid coupler...non-flexible. You have little to no room for error on the alignment or you'll be chewing up spiders and breaking the ears off the copupler halves. You also have to make sure you set the spider gaps correctly between coupler halves if they are too tight heat expansion will give you premature failures of the spider. I've seen what happens when couplers aren't sized and set properly. IT's ugly
Good luck with your project its giving me ideas I just might have to steal a forklift from work
If you have any questions feel free to ask. By the way you got that Metro before I could I wanted to buy it but that couple that had it wouldn't answer there phone!