I would use Coastal Enterprises PB BOND-240 glue. It is similar to Gorilla glue, a one-part glue activated with a spritz of water but does not bubble up or expand as much making for a neater job.
I did manage to get the first set glued and clamped up but not until after supper. :)
Tomorrow I'll remove this set of letters from the clamps, finish them off, weld them onto the sign and then start in on another six or seven letters. Stay tuned...