Would they be too small or should I just stick with 35 inch BFG MT's?

 

THe 305/70-17's are basically a 34x12.50-17 tire. They will look ok, but personally, I'd go with 35's if you have a 4x4. What wheels are you using. Some aftermarket 17 inch wheels will not fit over the brake calipers on 06 and some 05 year model trucks. Ford changed to a larger brake caliper during the 05 model year so some aftermarket 17's fit, some don't.

 

For the 35's I would like to use a 17x9 pro comp rim. I like the 1069 or the 6079 pro comp rim but I dont know if they will fit my 06 fx4.