A couple of decent size locking C clamps would hold that plate to a chassis rail without drilling any holes in the lower plate and would pull it down for your welds further up as well. Move them along the plate as you weld it to keep it tight down.
Bloody hell Cris, you really do get everywhere!
hahahahahahah
says YOU!!!! lmfao......
I've told you before, I'm God, I'm everywhere.