I use offset bushings everytime for this exact reason. I've done alignments for over 40 years. Many times even with the bushings ideal caster is not obtainable. You also have to remember that any chassis rake effects caster, rear end up reduces it
Additionally think about what happens to the toe pattern when you add too much positive caster. It lowers the outer tie rod and can create an undesirable toe pattern. Personally 2.5 degrees seems about right. My 64 Belvedere drag car regularly runs close to 150 mph with stock parts and 2.5 degrees without issue. This was achieved with off set bushings in the front location and shortening the strut rods .375.
Doug