I have 2 greyhawk myself both in 54 cal.T/C makes a scope base that fits the greyhawk. One screw goes where the rear sight is and the other at the back of the barrel. You will see a screw there .I have a simmons 1X-5x on one and a leupold 2x -7x on the other. Both of mine shoot the 54 cal 348 gr powerbelt and 100 gr of triple 7loose powder exceptionally well.
.If you use triple 7 send to T/C and get the nipples for musket caps that are1/4 x 28 thread. The musket caps are a lot hotter.
A local gun shop has one for sale in 50 cal and they are asking $175.00 for it.So $150.00 is a good deal.