Hi, Richard, Go for the 50mm bore ones for all your wheels and have a spacer machined up, I have the dimensions here for the spacer. 50mm bearings cost about £6.00 each on ebay, cheaper if you buy bulk, 40mm ones cost £30.00 each and why not keep both seals in the bearing. I done my two rear wheels like this and they run perfect, see a previous thread on this subject