Well. I got out the FALs and the tape measure.
My long SA butt stock measures just over 11".
The Para just under 10".
Both the short Imbel and wooden Izzy just over 9". However, the plastic stock would measure about another inch.
About the Ace conversion. The only difference between the standard Para set up and the Ace set up is that the lower doesn't need any modification. The adapter block bolts right on and the folding knuckle attaches to the adapter.
I think that Ace has several lengths available.
To convert, you will need a para bolt carrier, a para top cover with the spring and spring guide. On the para, the recoil spring runs through the top of the bolt carrier.
Headspace should be checked after changing the bolt carrier, it's not supposed the effect headspace, but I'd check it anyway.