There's enough slack created by loosening all the main bearing caps to pull the upper seal out with a big sheet metal screw threaded into it. The tricky part is getting the new seal pulled into the groove over the crankshaft. Lisle Tool Co. used to make a "Chinese Handcuff" tool to pull the new rope seal into place. The success rate for stopping a leak with a new seal was around 50% in most of the shops I've worked in since the late 1950's. There's quite a learning curve involved in getting it right most of the time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!