Hydroboost units have a built-in accumulator. It's good for about 2 pumps if it's fresh depends how much you module the pedal as you use it.

I did more digging on that part number and it's just confused me a bit more. It may already be 1-1/8". It may be from the 67-69 camaro time range, or it may be from 77-82 corvettes. GM seems to re-use that part number somehow. Unless any certain part options pop up I'll just measure the specs on the neck, piston and pushrod seat and make sure I find something compatible from spec from some vendor or NAPA listing.