I assume that you are a tractor ride guy. Remember that there were two transmissions used in the 880. Early had spur gear and the late had helical gears. The Oliver Shop manual shows 13.05 for the row crop helical gear trans, spur gear is 12.86.
The Industrial 880 is 16.8 MPH in 6th with 14 x 24" industrial tires.
Easy solution, find a complete 880 industrial transmission. Install the axle housings from the row crop transmission, lets say with 13.6 x 38" rubber and that will add to your speed by 1.3 or to 21.8 MPH.
If you really want to go past the red boys and leave them in the dust, there are four gears from the industrial trans that you can install in your row crop housing. Now the industrial has a 5.66:1 ring and pinion ratio and the row crop has 4.70:1 ratio. Parts from the industrial, in your row crop housing and the larger tires and you will be at 26.16 MPH, hummm wind the engine up a little, if there is any breeze you are driving into guarantee your hat won't stay on.
Been there done that. If we are talking about the spur gear trans you only need to change two gears.
The other green guys with their two shaft trancmission just have to make two gears that are in a housing between the two shafts behind the flywheel. Case guys change the sprokets in their chain drive finals.