Back half help

Whats up! I am trying to get some help on building my back half, I know how to build it cuz i have already made one but i am having trouble keeping it square. I got the back half built all together and measured and leveled everything to make sure i was on point and i was. When i went to connect it to the frame i got every thing squared up and welded it all up but for some reason the frame shifted real bad to one side and is all tweaked. I dont know how or were it bent cuz i had cross members tacked in it and had it tacked down to a 4x8x3/16 piece of plate. now i do know that when i started to weld i dont think the plate was thick enough because it bowed up and thats prolly why my back half tweaked. but i have seen people build these in the garage on the floor so im just wondering if you can give me any tips or trick as to how you would build one without a frame table cuz i dont got the money for that right now. thanks