Yes the AF covers will interchange between all the 7/8-series saws you've listed. The decal indentations may or may not not line up perfectly (cover and tank top) between the earlier 740/840/790/890 and later 795/797/895 saws, but the cover will swap on and function fine.
Is the 840 for sale? I been hunting for one of those with no luck yet.
Nick
Yes I could Joey since my 87cc 380 is a no go.