Quick google search to an AMC forum, that I've never seen before, indicates that the factory rear axle width is 58.5". That is conveniently the same as a G-body. So that makes for some readily avaiable after market stuff. Depending on the mounts, the sloppy Exploder 8.8 swaps would work well also.
Good thing you found that before I. I love the 'dare to be different' builds. But I'd leave the 4L60 alone and throw in a 4L80 with a turbo
check to see if the 304 is good first. If it's not, find a 360 or even better a 401. Both were used in Wagoneers. The 304 wasn't a bad engine and makes a good 'driver' if that's what you want.
AMC brings a very small audience but who's collectors will pay dearly for rare and original cars. and a 1979 AMC Spirit 304/ 4spd if original is a Very rare car. If you LS swap that car it goes from worth something to worth virtually nothing. It's also not an AMX variant which I think makes it even more rare, although the AMX treatment makes them look much better
You'll have to build your engine mount frame stands and the trans crossmember, but other than that it should be a very basic swap as far as fit, can probably find a header for something else Chevy that will either fit right in or fit with a small tube modification.
You're not going to find an motor mount "swap kit" available , to have this just be a bolt-in deal....to rare, not enough market for a kit..
IIRC it is a fairly big open engine bay for a small car, much like the Fox Mustang bay.
It's a leaf spring rear, so an 8.8" Ford swap is a piece of cake. These have a surprising amount of rear tire space and large inner wheel well...a 10" rim fits with the right offset and a 295mm tire is no problem.
Click on above and scroll way down to the specs for 304 V8 4 speed car. The engine is apart most likely because it had issues. Maybe serious, maybe not. Try to get the 304 running without too much cost or effort if possible. Drive it for a while and see how it runs. They were never a power house engine (125 HP). Heavy engine, more than a SBC but less than a BBC. Good low end torque and didn't rev much beyond 4500 RPM. That year would have an electronic ignition that worked well. Carb was a Carter 2 barrel if I remember right. The carb will most likely need a rebuild. I had the same engine in a 1977 International Scout II. Ran well and dependable but slow.
If the 304 is junk, runs poorly or you don't like it, I would put in an aluminum block LS1 5.7L (350 rear wheel HP) from an F body GM (Camaro or Firebird) with 5 speed manual over drive trans. Try to find a donor car and get all the wire harness, computer and accessories. A local guy did this to his Spirit and it really woke it up. The Spirit already has a front end to handle a V8. The rear end will need changing if you really jump on it. Again, the rear from a donor car could work. Fabrication will be needed for engine mounts, trans mounts, drive shaft, rear end swap, Unibody frame connector/enforcements, exhaust, radiator mounts, accessory mounts, wiring, etc. If you don't have the time, tools, garage, money, skills, experience, patience, dedication, persistence etc., don't do it. This is a good sized project but doable. From your pics, the car looks in decent shape. Check for rot in the floors, trunk, rocker panels, etc. Good Luck. Let us know how it goes.
Locating a rebuildable 360 ANC core and rebuilding that is probably going to be the most economical. Probably close to the same as rebuilding the 304, but with a lot better power potential and street manners.
LS swap is going to get you on all the little modifications, wiring, fabrication to get it all working, not the initial motor purchase price.
The 360 just drops right in place of the 304, wires connect right up, trans hooks up and will be much easier to upgrade to a better manual trans when it does blow up.
360 seems like more money up front on the rebuild, but no real expense after that, it bolts right in.
Or you find another used AMC V8 engine of any size in good enough shape to just clean-up, re-gasket, and toss it in....this will be the cheapest route
Rear axle should be the AMC Model 20, which is fairly stout, close to 12 bolt Chevy territory and at least 8.5" Chevy 10-bolt strength. It will have 2.87 gears and may well have a limited slip in it.
If you do go LS, even the little 4.8 nearly doubles the factory HP of the 304, and a 5.3 easily doubles it.
Whatever you wind up doing, please throw some updates out on this Small, lightweight, pre 2000 US chassis are rare. Would love to see the final result. Also, don't discount what can be done with a NA truck motor. 6.0's can easily put down 450 (400whp) NA with headers and cam. 400hp i n a little car is beyond fun.
Decent 304's can be found in the Jeep CJ crowds. The 304 was an emissions engine. The 360 and 401 were performers kept to the big Wagoneers after the gas crunch. A rebuilt 360 goes for about $2,500. Engine for a '79-'91 Grand Wagoneer will get you a 360 pretty easy