I have never seen 8 bore barrels marked as 3/4 choke. Surely the markings refer to a 3 & 3/4" chamber length? The word "choke" just means exactly that and replaced the previous mark of "not for ball" or 8B 9M etc.
As for using Eley factory 3 1/4" card and felt wad loads in a 3&3/4" chambered gun usually means poor patterning despite the chokes.Original Eley cartridges are worth a bit to collectors.
If you are going to use it for "fowling you would get better results by loading your own in 3 1/4" plastic hull with plastic skirt wad by Guilandi and lead shot - but if non-toxic shot use bismuth. I would caution against using steel shot in it.
