Bill:

With antique 30 engines 60" might be a bit too much unless you build a lot of "Lightness" in, and then you'd be limited to relatively windless days. But your overall length percentage sounds good - on a twin I generally like to be a little longer so I can get more rudder authority in case of a single engine flame out. Using the "Pushme-Pullyu" this would not be a consideration, 80% is fine.

If you want a P-P Stick you have an easy route, buy almost any 40 size stick and maintain the proportions, chop the fuselage off a little way behind the wing mount and add the second engine. Then cobble up two booms to support the tail and you're done. The pair of 30s should fly a 40 stick aggressively.

Bill.