AFAIK; Magnum truck engine mounts are different from LA engine car mounts.
That might slow you down some.
Assuming you already have a 318;
the power difference in the 5.2, over the 318, is in the compression ratio, the better heads, and the bigger cam. The short blocks are similar enough that you could call them the same except your 318 already fits in your car.
If you were to pump up the Scr on your 318 and install the Magnum heads and upcam, then you would have the same or similar output as the Magnum. You just wouldn't get the roller cam, which IMO, in NOT a big deal.
But
There are better than Magnum, aftermarket heads, already available for the LA; and FTH cams are cheap.
The really big deal is getting rid of the FA hiway gears in favor of more performance oriented mid-3 series gears, like 3.23 or 3.55.
If you are unlucky enough to be stuck with 2.20s, then 3.23s would increase your TM (Torque Multiplication) by 3.23/2.2= a whopping 46.8%, making your engine feel that much bigger, everywhere in the rpm band.
If your FA has 2.45s then 3.23/2.45=plus 31.8%
Your next performance improvement, and perhaps this should be the first; is the TC (Torque Convertor).
The TC is so much more than a Fluid Coupling. It actually multiplies the torque coming into it to a higher level as it leaves. This ratio can be as high as plus 80% at zero mph, to as little as plus 8/10% in top gear at high speed. It does this automatically. And best of all, they can be tuned to start from a low rpm to at a higher rpm.
This is good to know because you can choose that point off your power curve to go from dead sucked-out to holichit is this still my old 318.......
Below is a 5.2 Magnum power curve.
Up the left is the torque scale, and the red line is torque output.
Check out the maximum available ftlbs torque; at WOT
at 1800, I read 270ish; this is 92hp ; the base amount
at 2200, I read 280ish; this is 117hp, or +27% over base
at 2400, I read 285ish; this is 130hp, or +41%
at 2600, I read 290ish; this is 144hp, or 56%
at 2800, I read 293ish; this is 156hp, or +70%
Ok so let's think about this. If your current TC was "stalling" at 1800/92hp which is dead sucked-out with 2.20 rear gears, then for a few hundred dollars you could be blasting off with horsepowers of; 117/130/144/156; or up to 70% more power .
But it gets better.
If you put those two together, namely stall and gears
Let's see how this plays out. Now I know your LA is not gonna put these Magnum numbers out there but the results when applied as percentages will approximate the same, so I'm gonna
down size the Magnum torque numbers to 70% to approximate the 318 power/torque.
Lets say your engine currently has an 1800TC, and
let's say you have a trans with ratios of 2.74-1.54-1.00, and lets say your car has 2.45 rear gears .
From the above numbers 1800 is 270 maximum available ftlbs, downsized to 70% or
189 in the LA. Then at the rear wheel ignoring powertrain losses, you could see;
189x(2.74x2.45)=1269 ftlbs. But at zero mph the TC could multiply this to as high as 1289x1.8=
2320 ..As the car begins to accelerate this ratio immediately begins to diminish to an eventual low of 1289x 1.3ish so say
1676.
But the torque is rising with rpm so by 3200 (earlier in the LA) it might peak at
(293x.7)x(2.74x2.45)x1.3=
1790
Ok so now lets swap out that slowazz TC and get rid of the slowazz 2.45s
Instead lets get that 2600 and 3.23s.
At 2600 we read 290 ftlbs, x 70%, downgrades to
203 in the LA, so on the start line at zero mph the new combo predicts;
203x(2.74x3.23) x1.8=
3234 ftlbs, diminishing to
203x(2.74x3.23) x1.3=
2336.. At peak torque, this would then be
(293x.7)x(2.74x3.23)x1.4=2541
Next let's analyze this;
currently;
2320@0mph, dropping to 1676, peaking @1760
Predicting;
3234@0mph, dropping to 2336, peaking @2541
By percent;that is plus 39.4%, plus 39.4%, plus 44.4%
And we never even touched the engine.
The penalty of course is your hi-way cruise Rpm has risen; 3.23/2.45=plus 31.8% . And that could translate to a fuel-economy loss of half that or 16%. Is the penalty worth it to you? Only you can answer that.
Still same 318 under the hood.
BTW
your current engine, according to the numbers I worked out in BLUE, At 2320 max available footpounds could possibly break the tires loose. But it doesn't have the sustained TM to continue beyond the chirp.
Whereas, with the new numbers of 3234 in purple, this should easily break the tires loose, and there is plenty of sustained TM that should carry the tirespin for a respectable distance.