Tire size and wheel width can also have an effect on steering.
I believe the stock wheels were about 5 1/2" and the tires were fairly skinny. If you are using oversize wheels and tires you may have contributed to the problem. If all the suggested mechanical issues are sorted out and you still have a problem you may need to do a PS conversion.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine