Advice on choosing speaker cables for vintage Altec Model 19 speakers

Use this, it's exactly what you want/need. Supra PLY2.0 - it's available by the spool. 

On high efficiency speakers look at Auditorium23, they are a cheaper take on Ken Shindo’s pure silver cables and simply fabulous

I have the voice of theater driven with 30 watt tube amp.used 12 gauge ofc wire from parts express and made own bananna from Amazon not cardas.they didn't have 100k cables in the 50 's. New crossovers better money spent better yet put a sublime accoustic electronic crossover on it,fantastic. Enjoy the retro.mine was used by Elvis talks to me through my voice of theater.he has not left the building.that should get the party going or is there a difference in wire capacitance and induction and dialectric?

There are no right or wrong speaker cables if they’re made of wire capable of handling the current. There is no objective difference but to humour your train of thought, you should find speaker cables from the same era for the best synergy. 

When I used to sell those beauties, back in the late 70's, they were wired with 18 gauge lamp cord through a multi-speaker switch box and still sounded great. I would consider 16 or 14 gauge OFC cable from Monoprice, depending on the length of the run.