Your springs may not be the problem here
I have a 1955 Ford four-door sedan with a 302 V-8 and a C-6 transmission. At speeds above 55 mph the car rocks side to side. I replaced the rear springs, however the front coil springs are original and I plan on replacing them. There are heavy-duty shocks all the way around.
Should I use stock 1955 front coil springs or go to different ones? There is a difference in weight on the front because of the change in engine and transmission plus there’s the added weight for a/c including a Sanden compressor.
Assuming you went from a 1955 Ford Y block V-8 to the later 302-cubic-inch V-8, you will have lightened your car by 165 pounds. However, if you went from the 1955 223-cubic-inch six, to the 302 you will have lightened your car by only 75 pounds. And if you went from a standard transmission to the C-6 you will have added back a bit of weight, but not likely so much that it would affect the springs or the handling. And if you went from the original Ford-O-Matic to the later C-6 you will have most likely lightened the car by a small amount. The air conditioning compressor would be negligible as regards weight.
My hunch is that your problem lies with front-end alignment and steering. When you switched engines you may have changed the alignment of the steering control arm due to the lighter engine, which could account for your side-to-side movement. I would also check to see if your front anti-sway bar is in good order. As for changing out the springs, you should check the ride height of the front of the car to see if it is correct. If the front suspension is sagging you will want to install new springs, but if the height is correct according to your shop manual, you shouldn’t need to do that.