Jorn,
I agree with what Trace said, especially about the manuals. The most important of Buick manuals you need is the Master Chassis Parts & Master Body Parts manuals, which list all the part numbers used on your car.
The 75's were introduced in March, 1957, and have a number of differences from previous Roadmasters. Finned aluminum brake drums and an upgraded interior are two that come to mind.
Bear in mind that as a "C-body" car (Buick Super Model 50 & RM Model 70); Cadillac Series 62 & 60), yours is nearly 3" wider than the "B-body" cars Special (40) & Century (60). The bumper ends are all interchangeable, but almost all body trim is not. Grilles on Models 40 & 60 have 100 verticle bars, versus 104 bars on Models 50 & 70. The glass interchanges with all GM C-body cars, but not with any of GM B-body cars.
Don't make the mistake that many have made and buy the pretty NOS part for sale, only to find out it's for the "smaller" Buick. (Not that any of them are that small).
I've had my Model 73 Roadmaster since 1991, and rely heavily on the big Master Parts books when searching for parts.
Good luck on your frame off job, and keep us all posted; several people on the Forum have Model 75's.
Regards,
TG (Tom Gibson)
_________________________
Luck is when Preparation meets Opportunity.