That sounds like about the right amount of clearance for a Posie's slider spring.

I went to a Durant to gain some travel, got maybe 1 1/2".

Notching the frame 1" will help considerably.

You can add a spacer with no problems as long as part of the spring pack is captured within the crossmember.

New spring center bolts are available at any real parts house.
Get a new one and use that to clamp spring and spacer together.