looks like an early 70s 318, you could look (from below) on the block on the sides and see if you can find a casting date and the engine displacement, that would verify it. 904 is the trans that would come behind a 318. You could probably drop in a 318 or 360 relatively easily, the 360 has slightly different mounting ears up front, and uses a different torque converter in the trans. There was also a bigger trans often used with the 360, called the 727.
In addition, a 727-trans is normally a bigblock transmission, but they did sometimes also come with a smallblock-bellhousing boltpattern so they could be bolted to smallblock engines. There are 2 kinds of 318 engines in the Mopar-world, the regular smallblocks, like yours in the picture, and then there's the poly 318 which, to my knowledge at least, always used the 727-trans behind it. also pretty easy to indentify by the 'wavy' lowerside of the valvecovers and heads.
904 has also been bolted on 273 small block, the mopar small block before 1967 some informations here: http://www.allpar.com/mopar/318.html
It's an LA (non poly) small block Chrysler, but what displacement? The engine number is stamped on a machined pad just below the driver's side head. If it's a 318, it'll have "318" in the number. You can also decode the build date and (sometimes) what shift it was built on. It's very likely that's a 904 behind it. Decent trans, and is popular with drag racers when drastically rebuilt. A 727 is better stock, so if you come across one of those for a small block, grab it. You can drop pretty much any 318 in there. As noted before, 360's are externally balanced, so if you look for one of those, try to snag an engine/trans combo to make your life easier. You can also put a pre-1967 Poly 318 in there and look more "period correct" for an old hot rod. Motor mounts line up, though you may have to drill an extra hole in the p***enger side (I did when I swapped the other way on my '66). Bellhousing pattern is the same across both small blocks. Later model Magnum 318's also have mounting ears, so you could grab one of those out of a newer truck and get an overdrive trans. Mopar makes regular carbureted intakes for those now, so the change over isn't too bad. The major difference on those are the heads- they flow really well, and have pedestal-mount rockers. Old intakes don't interchange to the new heads without adapters or hogging out the holes (the bolts go straight down into Magnum heads)
70-74 318 is correct. It has an aluminum water pump which makes it post 69, but the valve covers do not have the mount for the emmisions **** that started coming out in 74'.
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