MNTGUN...I do it the same way but I use those little chain comalongs...they are shorter.
Where you are now...I would use draw down wedges on both sides if I had to....
BUT...I wouldn't....I'd weld two lugs on the sides...then loop a chain thru that and up over the top of the plate.
Put a hydraulic jack on the plate (with a U shaped adapter for the chain to run thru)...run the chain over the jack....pump up the jack till it bends the plate down and you got it!
You'll need wedges for the last bit no doubt. Just cut 1" or so holes on 1/4" plate ...tack it to the sides...run the wedges in from inside to out....and you'll suck the plate down very tight!
I have tools I don't even know I own...