Sounds like a plan. Fresh gear lube of any SAE grade will start the flush. Drive it a while and then drain & refill. Over time you should have pretty clean internals and like I tell most folks, any lube is better than no lube. The countershaft cluster gear does a good job of oil distibution inside there. Plain chassis grease won't flow into the internal bearings well so there has to be some fluidity to it. Chassis grease has oil soaps in it that are fine for metal to metal oscillating parts like tie rod ends and ball joints but it's not made to lubricate gears and bearings that rotate at rpm. If the transmission doesn't leak a little, then there is no fluid in it. Some folks have an obsession about leaky engines, axles, & transmissions. Modern automitive components are made to seal better than an old slinger ring systems so they don't leak. If you want to slow leakage a bit then just use the next higher viscosity lube or get yourself a catch tray and put it under the vehicle. I use the catch tray myself and just let the old stuff seep like it was supposed to.
As for lubricants, one of the easiest "satisfy all concerns" is Sta-Lube GL-4 as long as NAPA will still carry the stuff. GL-1 will work in most conditions but it has no EP additives at all for heavy use. Truth be known, modern GL-5 will probably work in most conditions unless you have an overdrive transmission. GL-3 hasn't been available for many years and that was mild EP gear lube that was recommended for all the overdrives back in the day. GL-4 replaced it and still has a mild EP rating but it is getting harder to source every year.
