The '73 rear body area can be reworked to get close to the original, and MPC thoughtfully includes the '70 Convertible boot to dress it up.
Like many old coots, I'm probably repeating myself, but the "73" is really the '70 with a new body, chrome and glass.
The rest of the kit is straight up 1970 Cougar, makes a great donor for your '69-70 MPC rebuilder or resin, or the somewhat less detailed but ultra-common '69 Eliminator.
The chassis appears to have been stretched by a tiny fraction (I need to measure) but nothing that can't be solved with some actual modeling. '70-71 Cyclone stock car kits (ex-MPC) will have the 5-slot wheels from the '70.
Only watchouts are the integration of the transmission tunnel - you need to zap the MPC raised area with the Dremel, but clever paint and the position of the exhaust will cover the bet. Also, and this is a pet peeve with '69-73 MPC Mercurys and their nicely detailed engine compartments makes the front end sit like my friend Bill Lee's Eliminator did when he would nail the throttle on the 428 CJ with the 4:30 gears. The Boss 302 fits nicely, but will need Tim Boyd surgery (grinding on the top of the bellhousing and trans) to sit properly in the chassis.
In fact, I'll add that you could stab in the complete '71 Revell Mustang engine and chassis, then detour the leftovers to your AMT or MPC builder.