The optimum would be if someone has a set of plans for you. But barring that it sounds like you've got enough to come up with a plan that'll work.

I'm guessing that this was a poly or tip dihedral design. If so then it shouldn't be too hard to at least make up a resonable model. The tips would use the same rib spacing as the center section so that'll fix the span of the tip panels. Then to make things easier I would suggest that the tip panel spar be in line with the center panel spars. From there the ribs will set any taper that may be in the tips and now much, if any, sweep there is. And if you can find any pictures of the model that can also provide a guide.

Tail size isn't all THAT critical. It won't be hard to figure out a suitable size. The length is a goodly portion of the span and if the nose and tail are of average proportion I'd suggest you should have a stabilizer of about 85 to 90 square inches. Use the chord of the stab from your fuselage plan to figure out what the span of it should be. And although you don't remember it being built up I would suggest you make it this way to keep the back end lighter. 1/4 inch would be a good thickness with the "spar" or hinge line strip being from 1/4 x 1 and the leading edge from 1/4 x 3/4. 1/8 x 1/4 strip ribs holds the parts together. The elevator can then be made from 1/4 x 1 trailing edge stock or carved to a similar shape from sheet stock.

And I trust that you're not planning on using the 1991 motor and battery technology at this point in time. Lighter tail structures combined with our far lighter brushless outrunner motors should make this old girl really sit up and sing sweetly.