Actually, the spacing is different between 7 & 8 speed, by 4mm. You'll almost certainly need a longer axle to accommodate the extra freewheel width, as most 7 speed have little clearance to the dropout. Most people will just spread the stays without coldsetting them, but with a freewheel you definitely want to have them coldset and the dropouts realigned. The extra 4mm on the axle will cause enough bending moment without the extra stress of non-aligned dropouts.
Bent axles are a relatively freqquent occurance with an 8 speed freewheel set-up, You may want to consider switching to cassette. I'd stick with 7 speed if you want ot go freewheel.
7 & 8 speed freewheels are relatively rare, as by that point most manufacturers had gone cassette/freehub. It's not that you can't find them, it's just that it's much harder to find a good selection of cog ranges, due to the lack of demand.
SunTour and Sachs are both of of business so they'll be harder to find. Shimano and Sunrace are both good values, but limited in ratio selection. IRD has a slightly better cog selection, but are much more expensive. My personal choise would be Shimano. FYI, Maillard was bought by Sachs in the mid-1980s, who were taken over by SRAM in the very late 1990's, who got out of the freewheel business.