I would forget about Boyeson's for an RM370, I'd be cutting out the ribs in the reed cage and cutting out a full reed petal from carbon sheet with a pair of scissors. I did it to my RM125B that Ted now owns and it's the only mod to my '79 KX250 that has holeshot Evo Open class races.
For 370 case-reeds I'd be going for .45mm carbon sheet.