Actually, the intake width issue comes into play when trying to adapt an LA intake to Magnum heads. On a Magnum block, with Magnum heads, the LA intake isn't wide enough. On an LA block with Magnum heads, an LA intake fits. I remember reading about using LA heads and intake on replacement blocks, and using the LA heads and LA intake together will work (and I'm not talking about the intake bolt arrangement being a factor). The LA intake bolts to the LA heads as before, no problem, so you could use the stock TBI and intake. However, the issues come into play after this are another matter.
1. Rocker are ratio. The LA rocker arms are rated at 1.52:1. Production units are usually closer to 1.46:1, give or take. The 360 Magnum cam is ground to produce .410" gross valve lift (just like the 360LA), but on the Magnum, this is with 1.6:1 rockers. So, the 360 Magnum cam, with LA rockers will actually put you with less lift, etc......more like the 318 LA cam, and that's just plain bad. Solution? 1.6:1 rockers for the LA, or a new roller cam with the lower ratio LA rockers in mind.
2. Rocker lube. The LA's lubed via the #2 and #4 cam journals through the block, and up into the heads, finally into the rocker shafts. The Magnum blocks have these oil passages, but they are not drilled for use, so the quick solution is to lube like the Magnums do.....through the pushrods. They must be hollow, and I'm thinking you'll need to use the LA roller cam pushrods---the length is close, but not the same as the Magnum pushrods. Just make sure that whatever rockers you use, they have oil holes in the pushrod cup so oil can get through to oil the rocker shafts. The other solution is to tap the oil passages so the LA rockers can lube as designed. You will also need to get a roller LA cam (for the oil holes), or have the Magnum cam drilled to match the LA setup.
3. Use the ignition system for the RC, including the distributor.
4. The 360 Magnum is externally balanced, just like the 360 LA, but the weight is a little less on the Magnum, so your flywheel or torque converter will need to be rebalanced.
4. Use the RC belts and pulleys......less of a headache.
That's the bulk of it. Less difficult would be to use Magnum heads and thrash out some sort of setup for induction. Easiest is to do like I said in the first reply.