had to double check mine.i have cueball wizard and sf2 and they are 27" 
SF2 has the correct sized legs - that is 100%.
I guess the manual for the Stargate needs to be checked - has anyone got a Stargate they can measure ?
I've checked the manual I have for it but I can't seem to find any info on the Leg size... I may have just skipped over it though. But I do have the old legs still and as you said they are definitely 31 inches and have that short ribbing at the top. Definitely need replacing though, they are covered in rust and very wonky!
I've attached a link to some pics if anyone is interested (Not the best pics as well, on my makeshift esky stand lol). As you can see the body is not in the greatest condition. The Bottom panel probably needs replacing, its structurally fine but yeah if I rub my hand underneath the wood flakes off... The cabinet in general is structurally sound, just quite battered as you can see from the pictures.
Side rails are all rusted up but should be able to clean them up with some steel wool. I've tried looking for some replacements online but no real luck, the responses I have received have just told me it would be easier and cheaper to try clean them up myself. I'm not expert but I think the playfield is in reasonable condition... Hopefully with a good clean and polish it should come up pretty nice. All the gold coloured metal rails need to be removed and cleaned up as they have some light rust on them. The only broken piece is the plastic you can see in the picture but pbresource actually has one in stock!
The game itself actually works quite well. The only part that stopped working ages ago was the glider craft coming out of the pyramid. I have been doing a lot of research on what I should be replacing but if anyone can give me anymore information or links to what I should be replacing ie. all the coils, flipper assembly etc. that would be great.
http://imgur.com/a/glvKx
