Bones, you don't need to touch the drive gear or its location. Many of those HEI's have that u-joint, its no problem.Carefully measure from the tip the the dist shaft to the bottom of the teeth on the drive gear on both distributers.The HEI is about 3/8 inch longer.Shorten the shaft with a grinder or file(slow), then reprofile the end to look like the old distributer. Check your work ,making sure the length and tip are correct.Hack saw off the collar and dress it with a course file.It does not have to be finely machined, the factory machined bottom section locates it.Make a clamp or you can butcher up the origial to work( better to save the original) When you install the dist, its very important to have it fully seated into the oil pump.Cleaning up the advance should work ok.If you have a fancy timing light with total advance features ,set the total advance at 32-34 degrees with the vacuum advance disconnected, but setting the timing with a vacuum guage or on the stock 5 degree setting at idle will be fine on a stock engine.Have fun