On a stock blank, the first thing we do is:
Inlet the tang and barrel. Where the end of the tang's breechplug goes determines the touch hole or drum & nipple's location.
Where the touch hole or nipple goes determines where the lock goes.
Where the lock goes (sear arm in particular) determines where the trigger goes.
Where the trigger goes determines the length of pull (and with it, the trigger guard).
When the length of pull is determined, you know where to install the buttplate.
However, you have a kit.
So, inlet the barrel and tang first, then the lock. Adjust the drum (rotation) to time it with the hammer. I had a traditions pistol that required me to reduce the drum size just so it would mate with the lock plate. Guys in classes after mine turned them on the drum to reduce the diameter to attain the same result.