The surface of the tang should be flat across. The rear edge for 18th c. should be square, or shaped with small squared shoulders, then a spear point or elongated roundish tip, or pointed tip. This can depend on which type of rifle you'rer making. The square is very common, and easier to inlet. You must bend it to fit the profile of the wood however. You can leave it proud from the wood and file it down to the wood, but you will want no less than 1/8" of thickness left, and around 3/16", or even a tad more would be better. You will also need to bevel the tang if it is not already beveled. Not a drastic bevel, but a very noticable bevel. This allows for better fitting. The rear edge can be slightly beveled, or not, but it is good to have a very small gap between it and the wood. Maybe around .005, or just a tad more. This gap will help to prevent the wood from chipping from recoil pressures, unless you intend to glassbed the breech and tang area. That is heresy to some, but SOP to others. There are pros, and cons. Glassbedding is certainly not PC, but it does make for a perfect bed which can, I repeat, can, make for better accuracy, and protects the wood from oils, water, and solvents. Done right, it is invisible unless the barrel is removed. That's your call.