You can see where I cut this one on our 71. This is actually a used part from our parts car that I tacked on, and then decided to replace it with new metal. Like Steve said, the lip is different and so is the lower, rear part.
I prefit the new qtr to fit with the old wheelhouse in place, removed it, then I cut the new wheelhouse the same as this pic shows and tacked it in. Temporarily mounted the qtr again to see what was needed, removed the qtr. Made relief cuts in the lip at the sharp bends, bent the new lip down. Replaced the qtr and cut metal to make the wheelhouse lip extend to the qtr lip. It's a fair amount of work, just be sure to make the new lip of the wheelhouse wide enough that the wheel well molding screws will catch it. You need the screws to go thru both the qtr pnl and the wheelhouse lip, and don't forget to make sure the new wheel house is holding the qtr in or out the correct amount. You don't want a dip or bow as you look down the side.
Are you using fiberglass filler to seal the welded areas?