Sierra's 5'th has starting load of 35.9 max load of 40.8 for I4350.

Nosler's 5'th is 39.0 to 43.0

Lyman's 48'th is 37.5 to 41.5

Hornady's 6'th does not list I4350

I would start working up in 1/2 gr increments you should start to see some better groups around 41.5 to 42.5 gr and give a load density in the 85 -90 % range. 4350 usually does pretty well in the 308 based cases but like BC said I would also try some 4064 or maybe even some H4831 SC.

Personally I would rather stay with a flat base bullet in the 260, less of the bullet seating down into the powder charge.