Happened once due to the front latch not properly set many decades ago. The top flew off on the freeway, ended up on the left lane shoulder area. Got off the next exit, went over to where the top rested, put it back on properly latched. The corner got dinged, otherwise that top was OK.
Never a problem if the top is latched properly.
Check all screw related to the top latch and their matching parts on the body is tight. They had decades to work loose. If any questionable hardware-screw are found, replace them and use Loctite 242 or low strength 222MS liberally where needed.
The top does not come un-done even at triple digit speeds unless the two front latches are NOT properly seated. If the latches are not properly seated, top lift off speed appears to be in the 50 to 70 Mph range.
Bernice
Why do you say your top is more like a 73-75?
The driver's side always lifted up when I closed the door and was never really tight.
Any pics of the mod? interested in seeing how it looked....
When I first built my car it was quite heavy so I was doing everything I could think of to get the weight down. I think it was a good 10 pounds shaved with the vinyl top. I drove it many years with no problems but always wanted another solid top and finally got one and painted it to match the car. Excuse the cobwebs as the top has been hanging on the garage wall and didn't notice them till after I took the pics. Tavalin, the latches and hooks on your top look good from what I can see. I have no idea of how it could have got away if it was attached right. I don't have any stock rubber but I have some extra hangers and metal parts.
I think it was a good 10 pounds shaved with the vinyl top. I drove it many years with no problems but always wanted another solid top and finally got one and painted it to match the car.