Well the bike has close to 5000kms on it now (160hrs in total / 85hrs on current top end). I've got some time off work over Easter and the parts are all in the post for the rest of the bike so should be arriving over the next week or two. It feels like the right time to get a start on this change over.
I've spoke to Dave (dmcca) again and got his postal address sorted and informed him of the plans so all is well on that front.
I've still got to get some 2mm resin/rosin core solder as I can't be bothered twisting it up like I did last time and feel it will be quicker and easier this way. Hope to give the bike a thorough wash down Monday (stripped back to make sure the frame is clean). If time allows I might pop the pipe, drop the oil, drain the coolant and generally just get the bike ready for Tuesday.
I'm dedicating this whole day to taking measurements both of the 300 top end and the 250, and by the end of the day hope to have the 250 top end specced and ready to post by wednesday morning.
I'm hoping the C-clipper arrives so I can avoid the head aches when fitting the piston.
Just out of curiosity, as I'll be just snugging everything up to set the port timing, get a squish reading on the head, and CC the trapped volume, do you think it would be safe to just slide the pin in without the clips? The bike will then be pulled back down again straight away. Or is there a big risk the pin will slide out and cause issues? Looking for feedback again 
Thanks!