Rivets are not permanent. It is not a big job to drill-out rivets and re-rivet. Monel rivets are good for this type of application. If you decide to use threaded fasteners, it might be worth considering using helicoil inserts. Whichever way you go, make sure that you use an appropriate barrier-coating between the steel
and the aluminium; Duralac being one such coating. If you don't, the steel
will cause the alloy to corrode.
On our 40' boat
the boom vang
fitting and the goose neck fitting on the mast
are attached with threded fastenings, not rivets, with helicoil inserts. However other fittings ion the mast and boom use rivets.