Swift would be a good option for convertable project. The reason being its big back
But like what rahul mentioned there are two important things to be considered.
1) Pillars mods is definitely required when chopping off the top of the car. When the pillar mods are done, it will for sure impact the doors. So Pillar mods along with converting a four door car to a two doors is must. Again for Two doors, you need to mod the front doors accordingly to increase the length.
2) After chopping off the hood, Then what??? Depends of the mods that you plan, either a metal covertable Top? or a Top that is made of cloth(Similar to the black ones that we see in the factory fitted covertables).
Designing the metal Top along with the hydralic system will involve too much of designing and there are high number of chances that the whole thing might go for a toss. So you might want to look at the tops with some hard cloth for the top.
Next is the Boot space, since this project is a convertable, i am sure it will covertable top will occupy the boot and you might want to look at this as well.
Anyways these are my thoughts and i may be wrong
some sketches that i found online.