[Arran]

The 5Y3 and 5Z4 are comparable in current and voltage ratings, the 5Y3 is 120 ma while the 5Z4 is 125 ma, however the 5Z4 has an indirectly heated cathode whereas the 5Y3 is directly heated. I have a Canadian Westinghouse set that used a 5Z4, it was a metal tube, and was worn out, so I substituted a 5Y4 (I think) by adding jumpers between the socket contacts. I think a 5Y3 will just plug in as a substitute without any mods since the cathode is connected to pin 8 the same as the heater.
Some sources claim that the indirectly heated cathode of the 5Z4 may add more of a delay to allow the other tubes to warmup but I think it's probably marginal. Why the two different types? Every 5Z4 that I've seen was metal so maybe that was a marketing gimmick to sell it as something special as opposed to a 5Y3, both were RCA inventions.
As for the 6Y6 the heater current is much higher then a 6F6 but the maximum plate voltage is lower. I suppose you could substitute a 6Y6 with a 6F6 but I would not go the other way around.
Regards
Arran

P.S On the split ratio tuning does the tuning knob pull out perhaps? There were some 1933-35 RCA and G.E sets like that, you pull the knob out for fine tuning and push it back in for course tuning.