Clark and Marsh hold a slim, one-shot lead over Bobby Brent Jr. and Jimmy Holman, both of Columbus, heading into the final round. The two-time past champions sit at 13-under-par 131.
Steve Collins of Claxton and Wally Adams of Vidalia carded the low round of the day, 63, to move up into third place at 11-under 133.
Four teams are tied for fourth at 8-under 136, including first-round leaders Ben Benford and Bill Clanton, both of Albany. After opening with a 63, Benford and Clanton turned in a 73 on Thursday. They are currently matched by Rocky Costa of Alpharetta and William Bader of Roswell; Jim Kamis of Marietta and John Holle of LaGrange; and Al Everett of Cumming and Dave Sexton of Suwanee.
Clark and Marsh, who won this championship in both 2005 and '08, have yet to record a bogey in this year's competition, carding 13 birdies through 36 holes.
Of the 47 teams competing in the Championship flight, 39 still currently sit under par.
The Championship flight is playing all three rounds at the par-72, 6,475-yard Lakes at Laura S. Walker. The first through fourth flights are competing at Okefenokee Country Club - a par-72 layout playing to 6,412 yards.
The final round start at 8:30 a.m. today. The final pairing of leaders Clark and Marsh and Brent and Holman tee off from No. 1 at 10:20 a.m.
The low 48 teams based on combined handicap index comprise the Championship Flight. The remainder of the field was determined by lottery and flighted by team handicap index. Only the competitors in the Championship flight are eligible for the overall title. Entries were open to male amateur golfers who are at least 55 years old and members of a GSGA Member Club.