This is my first foray into the world of syncro shifters and 7 speed freewheels. I have 2 Dura Ace 7-speed freewheels, 12-24 and 13-26. I am trying to build a 12-28 or 29. The larger cogs are proving difficult to find. I'm wondering if 6 speed uniglide cogs will work.

I have read that some of the 6 speed cogs have integral spacers and am aware that those will not substitute properly for 7 speed because of the different spacing.

So here are my questions:
-Are the inside diameters and spline patterns the same between 6 speed and 7 speed?
-Are the cog thicknesses the same?
-When I find a used 6-speed freewheel that I am considering scavenging for parts how can I tell if it has integral spacers or separate spacers? (assuming that I am buying on BF or ebay where I can not easily inspect or disassemble it)
-Is the difference between the 6-speed freewheel and the 7 that the 6 speed has only the 2 largest cogs in the larger diameter spline while the 7 speed has 3?

Any other suggestions or helpful advice?
The cogs I'm looking for are:
16t and 18t in positions 3 and 4.
28t in position 7. (or 25t and 29t in positions 6 and 7)

Thanks,
Brent