OK, now I'm confused, which Dorman hangers did you get? Cause you mention the rear ones came with locking nuts, but at the same time you say the only hangers you have to bolt on are front hangers? Regardless, don't look at the hole size in the frame, look at the bolts you have - 10mm is too small for a spring hanger or overload bracket, but 12mm would be OK. If you were supplied with 10mm bolts put them away for a future project and drill the holes out to 1/2" and use 1/2-20 hardware. If it's Grade-8 then I'd go on the high end of the torque specs (which for 1/2-20 thread would be 110 lbs-ft) since you're losing some via friction in the locking nuts. If you got 12mm with your Dorman hangers you can use them (chart says 83 lbs-ft, probably a few more should be added cause of lockunt tho), but personally I'd still go the 1/2-20 route there.
I would advise against welding on a Grade-8 fastener. Instead use loctite if you think the metal locknut alone will not be enough, red loctite if you're really paranoid.