Rudder Gudgeon plate

I am trying to replace the rudder pin gudgeon plate on my Hobie 16 but cant remove the 8 holding screws
any helpfull advice please.
Thanks Gerry icon_confused
http://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=27540&g2_serialNumber=4

There is a backing plate glassed into the transom, each bolt hole has been threaded, you have to be very careful not to strip the threads when taking the gudgeon bolts out. Somehow, you have to get some PB Blast onto the bolt tails inside the hull, maybe by using the straw extension on the nozzle? Let the PB work overnight and try again next day. If you do strip the threads removing the bolts, you will either have to re-tap holes on backing plate, or make a "L" shaped tool to insert lock-nuts through drain hole and hold them in place while screwing in new bolts. In the past, I used an old wrench bent at 90, using electrical tape over ring end to hold lock-nut in place, the bottom holes were easy, the top holes took a whole lot more ingenuity.

When removing the bolts, back out 1/4 - 1/2 turn, screw back in, wait for metal to cool, repeat untill bolts are removed.