If you want to make an authentic set of duelers your life is a bit easier. Most "correct" dueling pistols were smooth bore anyway. The code of dueling forbid rifling from pistols as it did sights (anything more than a front sight blade). Most dueling pistols were made so that they could be pointed (still at arms length) and were as complicated in geometry as a fine shotgun, so much so that they became an extension of the arm.
To make a correct set of early duelers, such as the ones produced by Wogdon you will probably want to bore your barrels out to about 50-55 caliber. The barrels were generally swamped, that is with a curvatured profile to improve balance. 7/8" might be a bit small, most of the orignals were close to 1" at the breech, but 7/8" would probably be OK for 50 caliber. Most dueling pistols were not used with especially heavy charges, probably in the neighborhood of 25-30 grains of 3fg would be max anyway in a 50 caliber pistol. They came with, and without a hooked breech. The famous duel between Burr and Hamilton was fought with at least one pair of Wogdons in 54 caliber.
These early pistols were full stocked in a manner not unlike the classic Kentucky pistols, but generally without the nosecap, and usually in fine walnut with flat sided grips. Decoration on most true dueling pistols was very limited, aside from some high quality, but restrained engraving. And quite often the pistols went without anything more than simple border engraving. The truth is that the duelers constructed by the well known Brittish gunsmiths were so well built that they did not need any further decoration, their undeniable quality was all that the eye required.
4140 steel should be fine for the breech plugs, though a better choice would probably still be 1018 Cold Rolled. The only heat treatment that I would recommend (in the case of a patent breech) would be a color case hardening, or you could simply leave the breech in a soft form and there would be no problem. The one thing you want to stay away from in breech parts (as well as the barrel) is a hard brittle material. When you think about it the only thing between your face and the explosion is that breech, a brittle one can shatter like glass and you don't want to be holding it if this happens. Casehardning is the traditional heat treating technique for reasons that I'll not go into here. As far as fitting goes the breech plug needs to fit at the inside shoulder of the barrel and also at the rear face of the barrel. Any gaps will at the very least allow fowling to spread into the internal areas causing eventual corrosion and could at the worst make the gun unsafe to fire.
Most breech plugs will be threaded between 5/8 and 3/4 with fine threads. I try to make the plugs at least 1/2" long and usually more like 3/4". If you want to make a patent breech that is another story. Dixie Gun Works sells several books that deal with the subject of making muzzeloading rifles and pistols, in addition to books on dueling pistols. I would caution you on the parts that they sell, in many cases the parts are very crude castings requiring so much work to finish that it is often times less trouble to simply build from scratch. The exception is their machined breech plugs, these are very good parts and are a bargain in my opinion. Still, a better company is Track of the Wolf (look at their website it is
www.trackofthewolf.com) and if you really get ambitious look into their special projects catalog for a feast of special small run castings that will allow you to recreated just about any historic arm ever made.
I hope this helps. In reality there are far easier (and probably better ways) to build a set of duelers like your talking about. But if you have the time and the ambition I don't see why you can't proceed like your doing, the main thing to remember is to take your time and do a lot of research into the subject before you start cutting.
Best of luck
Alex