Lejo, the area where I think you might be surprised to run into unexpected difficulty is your rear triangle putting your rear wheel in the dead center of the plane of your frame (exactly between the chain stays and seat stays). I would be willing to bet a great deal of money that if you tig your chain and seat stays to your vertical rear dropouts using only your fixture to hold them in place, there is almost no chance your rear wheel will center. This is why horizontal dropouts were used in the classic era before those old builders were able to buy or make a decent fixture. The dropout screws adjusted the wheel to fix the issue. I don't trust my very expensive fixtures to do that. Keep in mind that a one millimeter difference in either stay length is magnified about 3 times out by the tire. This is why extra care has to be used or, as an alternative, choose much more forgiving horizontal dropouts.
Most builders (if they are using vertical dropouts) use a "builders wheel" before brazing everything up to insure the lengths are right. This is a wheel that is trued and dished very carefully for intermediate checks.