Welcome to Allpar. If it lifted on a jack or parked on uneven ground, you may not be able to open the doors. If all they did was remove the roof without adding reinforcement, it will flex. The roof and cowl were a structural member for hardtops as there was no B or C-pillar.
Is it drivable? I would check it out thoroughly.
If you can find a similar car to compare that is a factory convertible you will see what IC is referring to. My 1986 Lebaron convertible has all the reinforcements on the inside of the passenger area. The main one looks like a stretched sideways letter H with the uprights going across the front and back of the foot well area. This piece is probably close to 1/8" thick, the crossbar of the H is down the center and is about 2" above the normal floor tunnel for a K car. The result is a very solid topless car. According to my son, much solider than his 1999 Mustang convertible.