would really recommend that you pop to a weighbridge so that you know for sure what the boat and rig are weighing in at, remember hand built! did Mike weight it all? you are making some assumptions here and probably best to check so no surprises.
if finances allow then just get a decent trailer instead of all this dicking around with that item you have, you can then fix it and sell on or straighten and sell on. to me the trailer is so important and "if" the design is not good then what other things are going to crop up in time? make sure you get a trailer with reasonable reserve capacity if the choice presents it self, dont get something on the limits.....weigh your boat then get the appropriate trailer.....wasting time considering a 1100kg trailer if your boat weight is not certain is just silly
all this means lost time boating and there are few enough opportunities in this damn country as it is!
if finances dont allow then i guess you just have to fix it somehow, however if the trailer is modified as you are suggesting or attempting would this affect the insurance?, would it need to be recertified?. I know nothing about this area of things so could not advise.
also, do you know if the wheels, tyres, axles are correctly specd on that trailer?
in summary i suspect you have limited confidence with that trailer so just sell it on and get a better one, the cost to you is the delta between what you sell and what you pay, might not be that bad and save a bit of stress