I did this before and you do NOT want sudden impact 5mph vs 0mph, because that is basically completely uncontrolled.
What you want is controlled. Completely controlled power.
Cut some 3/8 or even 1/2 scrap plate to fit onto whatever you are pulling, sandwich both sides in that with lots of grade 8 bolts, tightened down to quite snug, weld a chain link onto the outter (from the dent) side, and then use a come-a-long to gradually apply more and more pull until you have the frame deflected to straight ... take the pressure off, resnug your bolts. Probably have to give it another tug going just past straight - when you relax the pressure it will suck back to just straight. Anchor the other end against some completely immovable object. If you just are not gonna win doing it that way use some judicious heat - don't get carried away, and don't pull 1/2 as hard when it is hot - it will move like paper when it is hot. Temper it with heat again and let it slow cool so you haven't made a brittle cracking point.
Pro safety tip, never stand along the path of the come-along/chain etc - if your weld fails or the chain snaps it will cut your ass in half.
Don't have all the tools to do it right, find a buddy with a farm shop or break down and take it to a collision shop.