The easiest way to do the swap is to change the complete hub units, both front and rear. The rear hub units are a straight swap, but you will have to remove and then refit the drive-shaft coupling (it won't go through the hole in the rear wish-bone). If the bearings sound good in the 126 hub (check prior to removing coupling), I have been advised that as long as you are able to obtain the correct bearing pre-load, you do NOT have to replace the collapsible spacer. I would suggest that the opportunity is taken to fit new couplings. With regard to the front hub units---I would suggest that you do NOT try and fit 126 hubs onto 500 stub-axles---the complete front king-pin/hub units are a straight swap between the 500 and the 126. The 126 brakes will give you a slightly greater brake area, and unless you are going to tune the engine, will be perfectly adequate. You will, as has been previously mentioned by Chip, be able to still use the standard 500 master-cylinder. If you are going to overhaul the 126 brakes prior to fitting them (a) if possible retain the original brake-shoe springs and use GOOD quality brake shoes (e.g. METELLI)--a lot of the 'pattern' brake shoes are too thin, and you will have difficulty getting a good brake-pedal
