well, based on experience, its a miserable job. they are one of the most awkward tractors to work on unless you have the right tools and shop equipment. to do the clutch it is likely easiest to pull the engine. JD calls it a 'PERMA-CLUTCH' it is a wet clutch and usually takes some special JD tools to properly set it after it is installed on the flywheel. i still have a service manual and i can have a look in it to refresh my memory. the ABCD trans is in the rear section and the quad-range part is in the front section. the 8640 was a better tractor with several improvements, cooling system, lube circulated to the front axle, better improved engine, etc. a lot of 8630's got the original 619 engine replaced with the 8640 better 619 engine. the engine s/n tag on the block should tell if you take the info to a knowledgeable JD dealer and you should be able to tell if it was updated. around where i grew up probably 50% or more of the 8630's made were updated with the newer 619. if the tractor was worked hard, the original 619 would fail eventually, usually at fairly low hours, sometimes covered by warranty. my dad had an 8630 and then an 8640. i spent lots of hours on them. they were not a bad tractor for their time.
i remember getting into a debate with a jd salesman at the time