Squatting with Knee Replacement

Hey man, since you are the most respected power guy I know I thought maybe you could help me with my problem. I am trying to add some mass to my outer quad sweep. I just started to incorporate back and front squats into my leg days. I split up legs into quads on tues and hams on thrus. Previously I had been doing squats on a vsquat machine. (similar to a hack sqaut)

Now I am squatting w a barbell and I’m trying to find a foot position that puts the least amount of stress on my rt knee which has a full replacement. I dont have a problem until I go over 300 lbs. I do the reps very strict with a tempo of 312 down to a point where my butt is pretty much on the back of my calves. First I tried wrapping tight and that did eleviate the pain but I feared I may be causing other damage so I experimented with different feet position. I finally found a position that doesnt bother my knee but it is frying my hams. I spread my feet very wide pointing my feet outward. Squatting on a smith machine I can put my feet way out in front and this also helps take the strain off the knees.

Sorry bout the long post, would appreciate any advice.