i did this a few years ago, i bought shims from somewhere that fitted into the frame but there was no chance of the cups going in. i found the perfect way of doing it was to measure the length of you're headtube and cut a piece of 0.7mm thick aluminium sheet slightly shorter than your headtube and roll it to make a pipe. then use a rubber mallet to insert it into the heatube. it takes a little time to get the pipe just the right size circumference so the ends are butted together and not overlapping but when the cups go in their nice and tight. i used this setup for years with no problems at all.
