Off the top of my head I think for the FA Cup proper its:
Jeff Johnson 7
Jackie Swindells 6
Barry Howard, John Rogers 5
Barry Whitbread, John Hughes 4
Gary Anderson, John Davison, Ronnie Ellis, Lee Poland, Andy Reid 3
Malcolm Bailey, Graham Heathcote, Bert Lister, Ian Morris, Own Goal, Rod Thornley 2
29 other scorers with 1
Going back to 65-66, there are 11 FA Cup goal scorers I don't know (mainly in qualifying in 65-66)
But other than that the 218 FA Cup goals scored since then (not including this season) are shared:
Jackie Swindells 14
Rod Thornley 8
Jeff Johnson 7
Hang on, that means Jeff has never scored a goal in qualifying - lets check:
1978-79 FAC R1 Southport 4-3 Johnson 3, Bailey
1978-79 FAC R2 Droylsden 2-0 Brooke, Johnson
1978-79 FAC R3 Spurs 1-1 Johnson (oh get in you beauty!)
1979-80 FAC3R Orient 1-2 Johnson
1980-81 FAC 1 Burscough 2-1 Johnson
I guess it's explained by the fact that AT MOST Jeff could have played in 2 qualifying round matches:
Home Grantham in 1976-77 (but he might have been cup tied for that one), away Runcorn in 1977-78 when Mal Bailey scored
The best individual scoring feat that I know of is Fred Tabener's double hat trick against Earlestown in the 10-0 win in 1963-64, with two five goal hauls Walter Jones int he 8-1 win over Nantwich in 1947-48 and Jackie Swindells in the 6-0 win over Scarborough in the first round in 65-6