In my experience...since the exhaust pipes are too short on the 500/550 bike to effect an actual suction (even with4-1 headers), these bikes never needed mainjet changes for pipe changes. In fact, even with roadrace pipes (megaphones, the real ones) used in the 1970s on them, the most jetting change ever needed was to cope with the constantly-low float-bowl levels that came from constant 8000+ RPM running. Then they ran so rich at in-pit speeds that they were difficult to clear out when the pit stop was over, blowing black from their pipe(s) until they cleaned out again.
Changing the MOST EXCELLENT intake tract of this bike is [first of all] a travesty IMHO, as it was Honda's final solution to the problems developed by a broad 4-cylinder engine creating so much air turbulence in the carbs at hiway speeds that mixing became tricky, and this change became the OEM solution for all Japanese bike makers afterward. This is also why so many changes happened with the 750 over its K0-K6 lifetime. The swap in that bike from HM300 open pipes to the HM341 baffled pipes played the other 'step change' part in that saga, with the change (in Honda's major market, the USA) in national speed limits from 70+ MPH to 55 MPH playing the major impetus in the evolution.
So...I'd suggest: put the pipes on, run it for at least 100 miles with new sparkplugs, then look at the color of the plugs. If they are chalk white, consider going up in octane (like to premium grade) and re-test. BTW: this bike runs on Regular with today's fuels: today the Regular burns at about the same speed as Premium did in the 1970s when they were designed.
