Firstly, I am not a qualified engineer so will leave others to answer with superior knowlege about bearing clearance, however I am very much of the opinion that today's engineering tolerances could only be dreamed of in the 50's and 60's and that its not worth getting too worried about such things.
I would assume normal clearance bearings would be fine. The difference of 0.020mm is less than 0.001 inch (unless my maths has gone wrong) much closer tolerances than I reckon manufacturers were working to 50 years ago.
Regarding shielded bearings, I always use them in wheels as they stop all the crud from getting into the bearing. In a gearbox though I would have thought the copious amounts of oil would be a benefit.
Although thinking about it, if you left the outer seal in place (on a shielded type) it would provide a second line of defence from the dreaded gearbox oil seal leak (guess where my A10 leaks from!)
only my 2p worth I'm sure someone more knowledgeable than I will be along to put you right!