"the main bearing is being enlarged to 0.020".
To empathise what chaterlea said, the accepted way of grinding the main bring is to grind it unto it until it is round, then measure it (it might be say -014 undersize) and make a special journal to suit, a standard undersized journal of the next size could be purchased and bored to suit (so for a journal of -014, you could bore a -020 journal).
It is common for the Timing side journal housing in the crankcase to be worn oval, in which case it should be bored out slightly to a larger diameter, in which case a standard journal cannot be used, and a special one will need to be made, to suit the case I.d. , and crank o.d.
The final line boring of the journal to size, to acheive the correct clearance is a specialist job, but an experienced machinest should be able to work out what to do, as said in other posts this topic is covered very well on other threads on this forum.
Depending in funds it may not cost much more to get new billet con rods, so you might want to look into that, by the time old rods have been crack tested, shot peened, sized, new small end bushed fitted etc it may come to more than 50% of the cost of new rods.