If you search Google using ".350 Griffin and Howe" you'll find a number of discussions regarding loads which should give you enough information to get started.

Were this rifle mine, I'd pick a current .35 caliber cartridge with slightly less capacity, use starting loads for that cartridge and work my way up to a level that I was comfortable with given the design of the rifle. If it regulates, you've definitely found the place to stop even if you could obtain additional velocity with more powder.