The trunnion bolts are independent of the bolts holding down the top. I think my next step is to remove all but one bolt, loosen the fourth and rotate the top until I can access the slots and enlarge them. If that doesn't work I'll try to lift the top up and get some blocking under it to give the access. I have a car scissor jack but no engine hoist (or place to store one).
Helps to know others have had same issue and solved by enlarging slots.