You’re probably going to have to move your purlins/furring strips down closer to the edge of the tin. Hard to make ice cream out of horse poo!
Edit to add: and you got to much time on your hands. Don’t you have a couple of young kids and an ancient Studebaker? And a Ford N tractor! Better invest in a wax factory, cause you got both ends a blazing!

Edit 2: and in a post I think you said you wanted to restore 2 or 3 tractors a year??
You mean the board that the tin sits directly on? Why? So it will screw down flat as possible? I can do that but I'm more concerned about cutting it so It is straight as I place them across.
EDIT: I have 3 young kids (6 , 4 , 6 months), a wife, a dog, a 110 year old Studebaker, a 1974 IH 200 4x4 , a couple vintage mini bikes, several Cub Cadet lawn tractors, Pennsylvania Panzer, 1960s Craftsman Minature Riding Mower, Gibson Model D, Speedex 1650, a few Jubilee & 601 series Fords, a Few MF Tractors, an Oliver 60 Industrial, Oliver 70 RC, Allis CA, Allis G, Allis Chalmers D-10, Allis D-17 Series 3, Allis D-17 Series 4, Allis 210, John Deere 455G Track Loader, 1936 Graham Bradley/Paige Roto Tiller, (2) restored Maytag Engines, 1936 Farmall F-20, 1968 Farmall 140 High Crop, 1970 VW Beetle Partially Restored (in process), 1968 Astra 2+2 VW Kit Car (needs restoration), (38) Farmall Cub Tractors, (1) Farmall Cub Lo-Boy with Fast Hitch, (1) International Cub Lo-Boy with Front Blade & Auburn Trencher, (1) Cub 154 Lo-Boy, (1) Cub 184 Lo-Boy Parts Tractor, (5) Farmall A Tractors, (22) Farmall Super A tractors, (1) International Super A Industrial, (5) Farmall 100 Tractors, (3) Farmall 130 Tractors, (24) Farmall 140s, (2) Farmall C's, (1) Farmall Super C, (3) Farmall 200's, (1) Farmall 230, (1) International 240, (6) Farmall H's, (8) Farmall M's, (3) Farmall MD's, (1) McCormick Farmall W-9, (1) IH 364, (1) IH 464, (1) IH 404, (1) IH 584, implements to fit all of these tractors to include but not limited to about 40 complete sets of cultivators, 15 or so complete planter and side dresser units, plows, disks, rotary cutters, fast hitch equipment, snap couple equipment, several Covington Planters, plus all kinds of misc. stuff! That list SHOULD BE fairly complete as I keep a spreadsheet on everything as it comes/goes.