Prep motor for start.

Aw man... the old oil should be changed... all the fluids degrade over time; simple evaporation, slight moisture accumulation (especially brake fluid)... separation (metallic molecules in the oil sink to the bottom leaving the available oil for the suction tube low on 'cushion' chemistry like phosphates, sulfur, moly, zinc -the extreme pressure molecules etc

I'd say NO you don't wanna use a 'motor clean' type product*... not at first anyway until you see how the engine seems to be running -it may release too much sludge all at once -bear in mind modern oils have newer/better detergent anti-sludge chemistry (newer oils already have 'cleaning technology' in the oil)

Just put in new oil and 'give it chance' first IMO

Depending on how abused and neglected the engine is, IMO most likely several oil filter changes will be needed to restore good oil quality -the first new filter may clog on the first pass

all the 'cheapie' short oil filters like Autozone's STP brand filters are made by a good company 'Champion' but they all have an internal bypass... the internal bypass feature means it'll be bypassing the filter media as soon as there is about 8-10 psi back pressure from the clogging filter media.

Thing is, the old AMC inline sixes are so darned dependable -most likely it's gonna start and run; this deserves hunting for and spending a little more money on the better filters...

*I can almost guarantee it's gonna have carb problems if it's a one barrel Carter YF -be on the alert for flooding, which will indicate a ruptured/rotten metering rod diaphragm (mainly caused by fuel evaporation) -if/when that happens you'll get E-10 gasohol in the oil and that will work like 'motor flush' anyway (ethanol -a type of solvent, is a bad thing in your engine oil -causes loss of lubricity/poor viscosity)

...check to make sure the exhaust heating flapper is free on the exhaust manifold just below the carburetor -a few drops of oil on the spring and shaft will help/won't hurt

*IF it has a shaft mount rocker system EXPECT it to need an overhaul; the front rockers are first ones to get clogged oil passageways any they will clatter/'talk to you' (& you'll know) *the motor flush and/or ethanol solvent in the engine oil will not clean those oiling holes/no way to avoid it; must be done manually -poke a wire through the holes, clean up all the parts by hand etc. reassemble with plenty of oil or something like Lucas assembly lube

IMO, the previous owner may have run it 'until it died' and re-running it will reveal the reason why

Depending on how long it set up, sometimes they'll get a 'mysterious' stuck valve problem which will cause problems like bent pushrod and/or it'll 'run different' when cold or hot (the valve might 'free up' and 're-stick' depending on engine temp) -sometimes just 'bumping the stuck valve' can free it up and the engine will recover and run fine, sometimes not, in which case you'll wanna consider doing the 'rope trick' and work on the stuck valve a bit more; take the spring off and spin the valve using a drill! using solvent to clean out varnish in the valve guide/stem area

'75-up may have a clogged catalytic converter (typical) this'll make it run bad until it's removed; like having a potato in the exhaust pipe!

depending on how 'molested' the vacuum lines are... bug like to make a nest in the fuel tank vent tube -you might be able to run a wire in it and blow it out with compressed air (have someone else listen with gas cap off -be carefull doing this/be safe

Be prepared to get clogged gas filter real quick the gas is definitely settled in the tank and it'll reconstitute the sediment in the tank when you put new gas in

Be prepared for a ruptured fuel pump diaphragm -very typical on old cars that sat up for a while

Be prepared for a leaky/cracked rubber fuel line from the tank to the line -typical; they dry rot and form tiny cracks which'll make it seem like the car is running out of gas... (the fuel pump will suck air) + new gasohol needs new rated fuel line

-hope it helps -more AMC power to you & good luck!

No AMC odometer reads over a 100,000.  It starts again at zero at 100,000.