Frame size means nothing- providing it fits. I ride a variety of frame sizes and they can all ne made to fit. There is an optimum measurement though and that is Top Tube length. I like to have a certain lemgth of top tube because I have found that it works for me. 1 cm. either side and I can adjust for that with bar stem length and saddle position.

So the thing to work out is the Top tube length that suits you. Without going to a "Custom" built frame then a 68.5 cm. frame is going to be difficult to find--but that is not important. Measure your current top tube and look at various manufacturers to find a frame that has around the same length. This will be easier than you think as older bikes were short. Then look at Tall head tubes to find a bike that would be higher than you would normally expect. Seat heights are easily adjustable with longer seat posts so don't worry about that yet.

Material and you will probably be looking at Aluminium for cost but there are a few "Bargains" to be had that may get a better spec bike. Not that I like the bike warehouse deals but occasionally one will come up at a respectable spec for riding and at a cheap price.

One of which

http://www.nashbar.com/bikes/Product...0052_549659_-1

Don't get me wrong--I am not suggesting this is the best deal for your money but when it came up in my search--it is worth looking at. Largest frame is XL with a 600mm top tube. Compare that to your Miyata and see how it compares.