Truckedup.
Bearing catalogues publish the housing dimensions and the shaft diameter. The bearings are produced to run at pre -deternined running clearance and as long as the machining is done to the specified dimensions and within tolerance, that's it. From my Glacier catalogue for a big journal crank, crank pin is 1.6865/1.6870". Big end eye 1.8435/1.8440" Shell thickness 0.078" I'll leave you to consider the figures for max and minimum clearance, you may be fine with 1.5 thou.
Lots on the forum about the timing bush and the different types and approaches to sizing and accurate machining. Some folks convert to a needle roller type with modified oil feed to the crank end. Clearance here is based on shaft diameter, bearing type (pressure fed) and bearing material. Suggested figures on the Forum, and the bearing bore shrinks slightly when installed into the case.
Pistons will come with specification for the finished bore size suitable for the average intended use. This takes care of the running clearance. BSA published piston clearances for standard factory parts. Over time the metallurgy has advanced so better to use present day piston manufacturers' recommendation for new pistons. For genuine time warp parts, in their original model application,the BSA spec will do.
Looks like you have built a fair few motors and this one is no different in basic engineering terms.
Swarfy.