First of all, you need to make dang sure that you have the right bearings. There are TWO dimensions that come into play -- the ID for the crank journal and the OD for the rod big-end bore. One frequently sees oversize bearings on BOTH dimensions. Hopefully the box and/or bearings are marked correctly.
1) If your machinist has an accurate bearing mic, they can measure the thickness of the shell and get an initial idea if these will work. Measure the crank journals, measure the rod big-end bore, measure the bearing thickness and do the math. You want about .002 clearance. This is your starting point.
2) If these appear to be the right bearings, then it would be rare for NONE of the sets to fit. Usually I have to "tune up" a couple sets (with a rubber mallet), but not all sets.
3) I polish the outsides/insides with fine ScotchBrite and lacquer thinner. This is part of the process as well.
4) Make sure you do all the mic measurements - before you start working on the bearings themselves.
5) Checkout this little video I made a few years ago - might help you guys:
https://youtu.be/CyVLsPdbhS8?si=c8Qzhkq6HmtMDmLh
6) Also, some guys like "Ole' Ron" will open up the big-end of the rods about .001 just to give a bit more clearance. Keep in mind that these surfaces are bearing surfaces - so whomever is running the hone needs to be able to use the hone in a manner that polishes the big-end surfaces - as the bearings need to "float" in there. This is different than typical modern rods where the bearings are "tanged" and don't float in the rod.