What is a good replacement for my MF-Z012 6 speed wind on cassette?
1984 – Shimano introduced Dura-Ace 7400 series with Shimano Index System (SIS) the first successful indexed-shifting system in cycling history. It was a 6-speed (7 and 8-speed later) system with indexed shifters mounted on the down tube.

From http://cycling-passion.com/dura-ace-history/
Your front mech is given as a FD7400B, which doesn't really carry a defined number of &quot;speeds&quot; because 6/7/8 all used roughly the same chain width.  The B means it is a &quot;band-on&quot; fitting, not a F for &quot;braze-on&quot;.  It is rated for a big/small gap of 15 teeth.
Assuming your rear mech is from the same age, it will be a RD7400 which is rated for 6 speed and 26 tooth max cog.  A RD7401 is a 6+7 mech and a RD7402 is an 8 speed mech.
The downside is that this generation of Dura Ace had a whacky cable pull ratio.  So its ONLY compatible with a Dura Ace shifter of the same number of gears.  You already have both.
The upshot of this is that you need a 6 speed freewheel, where the big cog is no bigger than a 26 tooth.
If you want more gears, you'll need a new wheel hub, cassette, rear mech, and right-hand shifter.   Practically that means a replacement wheel.  And your frame has to have enough width to take a wider wheel - the OLD on your 6 speed wheel might be 126mm and wider cassettes use more space.  All that is covered in other questions on this site.
From Sheldon's site

Myth: Dura-Ace cassettes have a different spacing, and are incompatible with other derailers/shifters.
Truth: No, all Shimano cassettes and freehubs with the same number of speeds use the same spacing, and index with any system configured for the same number of sprockets.

You also need to avoid a freewheel with a bigger cog than 26 tooth.  The top jockey wheel could impact the big cog and that's loud at best, and disastrous at worst.
There's some interesting info at https://www.sheldonbrown.com/dura-ace.html  but you have the DA shifters and mechs, so that's not super-relevant to you.

Your biopace chainring won't care what you do at the backend.  Front shifting might get a bit twitchy if you put a thin 11 speed chain through a front mech intended for 6 speed, but it just needs to be pushed over a bit more before the chain gets shoved.  Easy.