Try a throttle block off a 76 or 77 340/440 Exciter. They used a 38mm Keihin roundslide carb and should have enough stroke to pull at least 38mm Mikuni's. They will be period correct for the GPX, the dimmer switch is the same and the oil pump cable went up to the throttle block so there are provisions for running twin cables. If you are running anything bigger than 38's you might fall short with this block and will then need to look at a 76 or 77 SRX block to keep a stock look.
Good Luck,
Phil (opsled)
