I would drill and tap the plate as soon as you have the plate let in, and you know the locations of your lock bolts. (like not put the bolt where the mainspring is, and on the bolster where it does not interfere with the bridle)
When you are fitting the plate to kiss on the barrel flat, it's mighty handy to be able to draw the lock in tight against the barrel with the screws. I like to get the fit so tight that a piece of cigarette paper will bind up. I will use lamp black or prussian blue to file the bolster to fit tight against the barrel. When in use, A light smear of grease on the bolster will keep all powder flash and water out of the mechanism.
Once the lock is in, then I know where the trigger and trigger plate goes.
I drill the tang screw after I get the trigger plate in. I sometimes braze a round bolster on the plate for extra thread engagement.
Tom