Before you get too carried away with taking anything apart,loosen the top cover bolts and yank the shift lever back and forth.As Joe said when in high the drivers side rail is slid ahead.The end of that rail slides into a hole in the bellhousing,and it only takes a couple of thousandths of being off to bind things up.If you shear off a piece of the new gasket things will bind up too.When in high the front and rear main shafts are just tied together,to make a straight through shaft.No other gears come into play,the shifter forks just hold them out of the way while in high.