As a minimum, I would install the ply rating specified by the manufacturer. In my case, the mains on the 185 are GY, 8.50x6, 6PR. I see no need for 8PR.

I think the 170 TCDS calls out 600 700 or 800 x 6, 4 ply.

I put 8 plies on my 140 because they were free, They are tail wheel tires for a crop duster 8 ply tires are heavy and stiff, they won't absorb shocks very well. I would put 8.00 x 6, 4 ply if I were you, unless the 8 ply are free