These images on the McMaster-Carr site show how I think the lifts would be best installed for this application:




I'd like the top to open to 65 degrees which I've shown in the model below (along with a crappy representation of the lift).

The frame is 31 x 47" and the top is definitely balanced in the middle. The top currently weighs 26lbs and could weigh between 40 and 50lbs once upholstered.