Oz-

The rifle looks like it's going to be loads of fun to shoot.
Get a load that shoots to regulation and you're going to have a great hunting rifle.
With that 6 Drams charge prescription you should get great performance out of it once you get it to shoot right.

I think that with a little help from our fellow forum members who shoot .577 BPE rifles, you'll be up and running as soon as you can fit the time in your schedule to get your kit together and out to the range for a few test-runs.

I can't wait to see how you do with this nice old rifle.

By the way, will you please show clear photographs of whatever engraving you see on the tops of the barrels at the chamber area, the serial number, and whatever stamps you see on the undersides of the barrels and the action flats.
Like you and a few other forum members, I also own and shoot a J&W Tolley BlackPowderExpress rifle. Mine is chambered in the somewhat obscure 500-450 3-1/2" Magnum Express, and is engraved with a charge prescription of 5-1/2 Drams.
Marrakai, another forum member here also owns a Tolley chambered for the same round, his is of a somewhat different configuration than mine is though.
There's a gentleman here who owns a very nice 12bore Tolley double rifle.

Further photos of your rifle would be greatly appreciated.

--Tinker