Just like the title says. Will a 72 cutlass convertible trunk fit a 72 cutlass hardtop?

 

The convert lid is narrower than the hard top. No.

 

In addition to the different width, the convertible trunklid has rounded top corners, the coupe has square corners. And the convertible/four door lid is "flatter" than the one for the coupe.

Note both the top corner shapes and the location of the bottom corners in relation to the tail lights.

 

3 different lids for 70-72 Cutlass IIRC.
1) Convertible and 4 door.
2) Supreme hardtop.
3) "S" and 442 2 door hardtop.
I think I screwed that up, it's close though....

 

You are correct, Greg, except that the coupe trunklid fits all coupes, Sport Coupe and Holiday Coupe. Depending on the model year, they were F85s and Cutlass (sans "S") as well as "S" and 442. Ironically, this is the one case where the part really "doesn't fit Supremes".