Just my opinion...but a flush on an engine that has been setting up for years, is not going to do much to rid the ole girl of that heavy sludge that cakes up the lower sump. If you've got the skills, pull the pan, use a putty knife (because that's what it's going to take) and dig out the grung. If the rockers are caked, it wouldn't hurt to carefully remove them, and off the vehicle, give them a good cleaning and flush. I wouldn't break a lot of stuff loose on the top of the head. You don't need that cascading down into the pan after you add the new oil. Just bolt the rockers back on, check her valve adjustment, replace the pan, stick in a new filter and oil'er up.
While you've got the pan off, it's a good time to check out the overall condition of the guts. I've found that a really gunked up engine may still give you some good service, but if it's cruddy on top and down below, you're probably at, or getting close to a rebuild. The thick crud is moisture, remnants of fuel from blow-by, and solid particles that are a result of the combustion process. The engine probably wasn't running that good when she was taken out of service. It's also a good time to knock out any dents or dings and square up the gasket surface.
A heavily gunked engine, to me, is indicative of
one that is reaching the end of it's service life before a rebuild. A clean top, clean rods and a little hard sludge in the lower pan aren't going to raise that much concern. Popping a rod cap or two can give you even a little more info.
If she's running, and the oil pressure is stable, you might be able to stretch a few more miles out of the engine.
Beltfed, hit it on the button about the old seals and gaskets. Even a 10 year old rear main that has been sitting idle, has the potential to cause you much grief. As the weakest component in an engine, you will probably wind up having problems with it even if everything else is clean and looks good. You may get lucky for a while!!!
If you plan on hitting the highway and running your little engine hard, you'd be better served, to do your overhaul before you bust something and wind up with a pile of useless junk.
