I know a magnum will directly bolt in place of an LA engine. And I have seen it done with either LA or magnum timing covers water pump and belt drive.
The question has to do with keeping working ac on a truck that already has it,
If using the LA front cover and multiple belts and a carburetor intake manifold, what ac pump and which bracketry is used?
All LA stuff? All magnum stuff! Some conglomeration between them? AC pump in question is the round aluminum in both cases. Not the old RV2

If the AC compressor is older and uses a v-belt pulley then your looking at the older LA setup for everything else I'd think. I suspect mix-matching engine brackets and accessories between the two types of engines could result in painting yourself into a corner as far as getting all the pullies to line up right.

If you use a 5.2 or 5.9 timing cover then you can't run a mechanical fuel pump if that matters to you.

I did a magnum swap in a 1991 Dakota and I just bolted all the accessories from the LA on. In my case the LA had a serpentine setup but it did have one less rib on the pullies on the LA. The LA had an air pump so finding a belt the right length is a problem.

I did the magnum swap in my 65 and had the aftermarket sanden compressor on that.
I took the magnum hose to the compressor and taped it to my sanden compressor hoses for length and took it to a hose shop in town to put the two together, so I could run the magnum ac compressor instead.
wasn't very expensive, 20 to 50 bucks at most.
works fine, if that is a choice.
most towns have a custom hose shop.
that way I could keep the serpentine belt setup.
I did make 2 changes to it though.
I used the van power steering unit instead of the truck one.
that changed the belt routing, so I changed an idler to a hemi idler which changed the dimensions of the belt back to a stock belt but more wrap than what I had before.

So i assume you want to use a magnum in place of the LA. What year truck/LA? Are you running a R12 or 134A compressor? Also what intake are you using and does it have provisions for the brackets? May take a look here for swap info.

Truck is 83.
Will become a 134 system.
Yeah I'm using a magnum long block.
Carbureted intake, not sure yet which.
That's gonna depend on what manifold I can find or else a drill fixture to drill my heads-or not.
It won't be the beer barrel as used stock on the magnums originally

there are bunch of cheap dual bolt pattern intakes that work on both LA and mag motors.
but none of them work with the magnum accessories correctly. so that limits you back to the la front end of it.
then you might need to modify or fabricate some brackets to mount your existing compressor to that intake.

there is 1 mag motor carb intake that works with the mag accessories, but it is around 500 bucks vs the dual bolt pattern ones for under 200. seen some as low as 120 bucks.

I've been looking at those intakes, between being guaranteed Chinese and the wavy pattern in the floor of them I'm just not sure that's what I want I have time but I'm looking for an M1 or an Edelbrock

I don't have any experience with the aftermarket intakes. Many years ago,I swapped an intake manifold. I think it was a stock 2 barrel for a stock 4 barrel intake. The one car had the old RV2 compressor, the other car had the newer style compressor. The issue I ran into was the outlet for the upper radiator hose / thermostat housing was moved slightly. I did NOT notice the difference, until it came time to mount the second a/c compressor. That compressor interfered with the water outlet. So, it's a great idea to check this before you perform the swap.

Be sure to use a triple pass condenser and a fresh receiver/dryer with the R-134.

Finding any condenser for those trucks is a pain
I just went thru that for my 85 now get to find another for this one