Not a bad idea Charlie many years ago (when Sylvia was alive) she was not interested in rallies when I had my angle cab truck she counted every machine on and when I came back she counted them again and I could not swap any because she knew every machine
rust in fuel tank is easy put some old nuts and bolts in the tank ( first remove tap if internal filter is fitted ) and do the conga ( shake it about ) recommended to treat tank with sealer when clean
why do you want twin wheels ? if for stability why not fit a pair of axle extensions which are relatively easy to find on flee bay know they fitted twins on Howard Gems
why would a concrete mixer need a two speed pulley ? every one I have used is single for drum rotation speed unless the engine has been replaced from something else