Here is a link on the centerboard:
Macgregor 26S Outside page 40
Macgregor 26S Outside Mods page 3
This is a 1990 model. Macgregor did make a change in the way the centerboard is mounted later. Your mounting bolts may be in the inside under the sink but these pictures may help anyway.
You can lower the front of the trailer as far as possible with the wheel crank, block the stern (with something solid - that won't move), then raise the front all the way back up. This will give you some room. (I then suspended my '91 with an engine hoist & pulled the trailer out - can be very dangerous if not done properly.)