king1010, Welcome to TBN, It appears to me that you have already bought 16x6 rims?? Are they tractor rims or implement rims? Either will work but I would suggest if you are going to use implent rims instead of tractor rims,that you shouldn't put ribbed tires on the implement rims..either use implement tires or car or truck tires. Reason is the tractor rims are made stronger for the torsional pressure the rim receives during a hard turn in a plowing situtation and the implement rims are not.
With all that said 6.00x16 tire should be fine for that tractor up to 6.50x16 should also fit fine .
I have implement rims with implement tires on my old 135 and it works great around here doesn't tear up the grass and gets plenty of steering traction..