You shouldn't need a different master with the adjustable slave. In fact you can swap the pin with a non-adjustable slave to make it adjustable. Sounds like it isn't adjusted right, or something else is going wrong. My fear is that if you use a 3/4" master it will either hyper-extend the pressure plate fingers or push the slave cylinder all the way out.
Your distributor looks like it might be getting some vacuum advance at higher rpms. If you have the stock 280ZX distributor and take the vacuum advance off or lock it down, then you should get 17 or 18 degrees mechanical, and you can look at the bottom of the plate with the centrifugal weights and it will tell you which one you have. When I was running an L28 with triples that's what I did, and just set the timing for something like 17 or 18 degrees at idle to get mid 30s at all in, which I think happens at about 2500.