The '55 first mounts just as you suspect. If you can find one, try to get the springs too. They will have the locator bolt in the right place. Your '54 locator bolt is too far forward. I have read that you can drill a new hole in your '54 springs but I don't want to try to drill a large hole through 18 springs.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.