The short answer is yes. There are a few caveats. It depends on the cam. The LA roller cams will physically swap into a Magnum. It is said the LA cam will come in contact with the timing cover on a Magnum, but I have not personally seen this to be the case. Probably some cam maker out there with that issue, but the TBI Magnums are slightly shorter since they were used with electric fuel pumps, so again, the timing cover clearance isn't a common problem. Now, if you're talking about flat tappet LA cams, remember that you cannot use these cams with roller lifters, nor should you mix roller CAMS with flat tappets. Looking at the stock LA roller cam specs, using one with the increased Magnum rocker ratio doesn't do any good. Take the LA 360 roller. Gross valve lift is .410" with a 1.52:1 ratio. The Magnum 360 has the same gross lift with a 1.6:1 ratio. So, using a 360LA cam in a Magnum would produce .431" gross lift. You could use the 318 Magnum cam, which has .432" lift with 1.6:1 ratio. So, unless you have an aftermarket (meaning more lift than stock)LA roller cam laying around, or can pick one up for cheap, there's nothing to be gained by doing it.
Well 50 bucks is 50 bucks, so why not? Just remember to take the LA rocker ratio of 1.52:1, divide your gross lift by 1.52, then multiply by 1.6 to get the gross lift for the cam when used in your Magnum. An LA cam with .450" lift will produce .473" at 1.6. Take that into consideration when replacing your valve springs and considering cam performance. Also you will note a hole drilled in the #2 and #4 LA cam journals. These are for the cylinder head oiling on an LA. Magnums oil via the pushrods. These oil holes are not a problem in a Magnum setting. Just install and fuggitaboudit.