yes and no.
Swapping a small-block Chevy engine into fox Mustangs was all the rage until 1984 (which is when it started to taper off quickly, and disappeared completely as a popular option by 1986).
Finding ready-made parts for such a swap isn't easy, but the engine will fit dimensionally if you make your own mounts or have them made for you.
All of that said;
The Chevy 305 is junk. While it has the same stroke as the more popular 350, that's about all they have in common other than external dimensions. 305 cylinder heads and blocks both have design flaws that leave them vulnerable to cracks, and you will be VERY limited in camshaft and cylinder head selection with a 305 due to their small bores and lack of piston-to-valve clearance.
If you are absolutely certain you want to build a Chevystang, go with a 283, 302, 307, 327, 350, or 400 engine. The Chevy 305 was the only real bad seed of the small block family, though there are the red-head-stepchild 262 V8s, as well as the original 265ci V8s (hard to find, lacks all the improvements that were pretty much defined by the mid sixties in the other engines).
Just remember that Chevy guys will laugh at it because it's a Mustang, and Ford guys will scoff at it because it's got a heartbeat to go with the hoofbeats. And a Ford 302 swap is a lot easier.
Also, if you're in an emissions-testing state/county/municipality... forget it. You'd have to swap everything from the Chevy... computer, every INCH of wire, all the sensors, you'd have to have a four-cat H-pipe engineered for it, and you'd have to completely fabricate your own fuel system since that El Camino is either computer-carbureted or Throttle-body-fuel-injected.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
