For my boats, which weigh around 100 pounds, I use two cords, one for each end, and one cheap boat winch, hanging the boat from the ceiling, using pulleys to route the cable. This is the kind of winch used on a trailer to pull the boat up on the trailer.
A few comments:
1. I have to be careful not to let the winch get away from me, and that is with only 100 pounds. I would be scared to use that system for 300 pounds. This type of winch is not intended for lifting, no runaway brake.
2. It is hard to get the boat to come up evenly. The two cords wrap differently on the winch, and then one end or the other is higher than the other. If you have plenty of headroom, it might be OK to have one end higher than the other. Sometimes I have to lower the boat and try again. You might consider a single lift cable, lifting from the center of the shell, to avoid that problem
3. I used eye bolts screwed into the joists. This is not a great design, as eye bolts are not intended to take a side load. I suggest that you use some angle to attach to two joists, use lag bolts to attach angle to roof, and then spread the load between two joists.
A hoist would probably be a better choice for 300 pounds, consider a cheap electric winch.
My two cents, based on ten years of using this system.
Richard