I just bought a new AB Biller 54 Special and plan on using it to line shaft in the Florida Keys, It came with a 5/16 shaft and 2 9/16" bands. What would be the best band combo for max range without over powering?

Add a third 9/16?
switch to two 5/8"?
or switch to one 3/4"?

Would any of theese combos require a shock cord that is longer than the one that came with the gun?

Two 5/8" bands should be plenty and the amount of line on the gun is fine but you may want to consider switching to a mono shooting line instead of that shock cord Biiler sells their guns with.

24" 5/8 bands will fit that gun nicely.

2 x 5/8 will work perfectly. I tried 1 x 3/4 for two trips and regretted not buying more 5/8 material.