My A tank has been tapped 7/16 BSF sometime in the past.
What I did was make some studs which are 7/16 BSF wasting down to 3/8 diameter with 3/8 BSF the other end. The 7/16 BSF part is slightly shorter than the depth of the tank mounting holes so only 3/8 diameter protrudes from the tank. I also on the 3/8 end slotted the end so a screwdriver can be used to screw the studs into the tank. The method of fitting is place the two mounting rubbers over the 3/8 part of the studs and then hold them on with a 3/8 nut and washer. The nut and stud are drilled for wirelocking and I temporary put a piece of wire throught them. Then position the tank on the frame, lift the front and slide the front studs in the slots in the frame with the rubber mounts either side of the slot bracket. Then lower the tank onto the studs screwing the studs into the tank two turns. Repeat for the rear mountings. Then with a short screwdriver tighten the studs. I don't believe the studs / bolts should be that tight and using a screwdriver I have found is tight enough. If the studs are the correct length they will nip the rubber mounting washers before the 7/16 section bottoms in the tank. Then remove the temporary wire and wirelock the two front studs / nuts together across the frame and the rear two together. It's easier than it sounds.
It is important that there is clearance between the studs and the slots in the frame otherwise you are bypassing the rubber mounting washers and vibration will be transmitted to the tank.
I found on my A it was not possible to screw the studs into the tank first and then fit the tank on the bike.
On a final note, my tank would not fit flat onto all four mounting points, so I made an extra 1/16 thick rubber washer for the offending rear mount. Then the tank sat on all 4 points and tightened down without any stress been applied. Something to look for.
Cheers,
Simon