We use two fiddle blocks (one with becket and adjustable cam cleat) hardware parts on our Tartan 27' for the main end boom sheeting. I am pretty sure that 7/16ths line works well for us and is good for handling.
If you have to replace the main sheet blocks & tackle DO check out Garhauer for additional parts. Garhauer Marine Hardware -943260
Usually much cheaper then Harken or WM and good quality.
The Pearson 26 is just a slightly smaller cousin of my boat without a center board. Pretty cool old boat.