Sounds like he's still got the sump stuck to the rails.
Get a flexible putty knife (buy one or two if you don't have any - a 2" usually works, a 3" is helpful in other situations) and get it in between the sump and the rails. Don't get the stiff ones - you'll have to work around a bend!
Once you get the knife edge driven into the sump seal, work it along up and down the rail to break the factory seal (it's silicone, and it's mashed down really well.) You can also pull on the knife handle to try to prise the sump down from the block skirt to free it up.
Once you have the sump loose, you may need a bit more room to get it out past the pickup and the bottom of the bellhousing. If you've got the front end up to full droop, make sure you have about an inch of daylight under the tyres - you can use the OEM tyre changing jack on the axle tube and front subframe rail to force it away. If you still need a bit of room, remove the four nuts for the transmission mount (13m/m socket, M8-1.25 if you want to replace them. I use Nylon collar locknuts here...) and lift the transfer case up with a floor jack - that works out to most of another inch (which is about what you need, most likely.)
I've done several sump jobs without having to disconnect anything in the front suspension, so I know it can be done.
Given you're working on a 99, you should already have the one-piece moulded rubber sump gasket (it was introduced in 1996, but fits all the way back to 1987.) You shouldn't need to use anything on it - I never have. Just torque the 1/4"-20x1/2" screws to 7 pound-feet/84 pound-inches and the 5/16"-18x3/4" screws to 11 pound-feet/132 pound-inches (yes, it's that important!) and recheck torque in a week. I like to use RTV on the screw threads as an ersatz threadlocker - it works better, and it's more flexible for retorquing in a week (which is needed with the rubber gasket, whatever anyone else tells you.)
Pay particular attention to the end rail going over the rear main cap - it likes to jump out of the groove for it. You can use RTV to retain it, but I've usually just pulled it into place with some wire after the sump is up and that's been quite good.
I find it helpful to install about four studs near the ends of the rails - 1/4"-20x1-1/4" (you can get a valve cover stud kit for SBChevvy with stamped covers, and that will work. You'll use half of the kit - save the other half for the valve cover when you do it.) Use LocTite #242 on these, and they'll let you hold everything up with one hand while you start the nuts with the other - saves a lot of wrestling later...
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