vintage Blown JAP 500cc in midjet speed car

[fitsracing]

G'day, want to set up WI between throttle and supercharger on vintage 500cc JAP hill climb cooper midjet speed car running on methanol.
the blower is a cabin pressure pump from a DC3. the problem with this set up over the last 50 years has been detonation,when changing gear. going from wide open throttle (WOT) max revs /pull and full boost to closed throttle for gear change,at this point high vacuum between closed throttle and blower, reopen throttle,still high vacuum and as surge of intake gas charge through open throttle until vacuum drops and the inertia of fast flowing intake charge momentarily gives a boost spike into and through blower, over boosting engine causing detonation.
too off set this previous safe tune obtained by retarding ignition and long intake runner.
wanting to WI so more ignition advance can be used .getting most performance safely.
this may be old school stuff with no electronics.
a waste gate or blow off valve would help.
but answer lays in WI, to get ignition advanced.without detonation.
can someone help?
regarding simple vacuum operated carby type WI,as the huge increase in manifold vacuum caused by closed throttle after full boost/revs.hoping to cure problem using the cause of problem.
the speed car has run almost continually since 1950s but no one has realised its full potential. I'm hoping
cheers tony