86 Black Roller 318, swap/interchange into 72 318

340 Forged cranks are balanced for heavy forged pistons. If you use one with 318 pistons especially hypers, it's gonna cost heavy dollars to balance that up.
IIRC those 340 pistons were around 719grams versus like 550 on the stock 318s.
Hypers would likely be less.
Save some cash, use the 318 crank.

FYI
My 367 has been running a FTH 230/237/110 cam, with lifts of 549/571, ever since 2004. Thus
the roller cam is not high on my wish list. Not sure I would use a Magnum bottom end even if I had one. Actually, as I think about it, no, if I already had a complete LA, I wouldn't bother with anything off a Magnum. Jus saying.
and
besides all that, a 360 LA falls together at about 10.7Scr with KB107s and 63cc Alloy heads. No funny machining required no endless calculations; Just balance the bottom end and screw it together. and the good news is that, with a proper quench, it will run up to 195psi (the highest I have tried) still on 87E10.. This would allow you to run any cam with an Ica between; 52* (the stock 360 2bbl cam), to 64* (the stock 340 cam), to about 68* before the bottom end starts to get soft; all of this runs on 87E10;
and if you deck it right, to get about 11.2 Scr, you can run up to a 72* Ica, all of it on a modest convertor and whatever rear gear (depending on cam selected), that you heart desires to run.
The savings of NOT having to buy/install a convertor and/or gears will more than pay for the 360 core and the decking. So yur into the project for the price of the heads, which are half paid for by not buying a roller cam; and they will be fully paid for in the following months by running the 87E10 over hi-test.

And now you have the freedom to make as many mistakes on cam-selection, as usually happens, cuz guys almost always choose too-big the first time.
The hi pressure 360 allows you plenty of freedom to make mistakes, that you don't have to come out a loser over..
I mean, for me, the decision was a no-brainer.
and the punchline is, that I was never sorry that I picked it out of my pile of cores.
Had I stuck with a 318, I'd still be kicking myazz, over 20 years later.

Listen; I'm not a 318 hater;
I'm a realist, in the long run, power for power, the 360 just costs less.
Look, you don't even have to set the cam-timing, just dot to dot it, and verify it's in at least close by laying a straightedge over the lifter tops; thus you'll save another hundred or more, and the 360 will have more power than your A-body chassis can handle, even if the cam-timing is out by up to 8* advanced to 4* retarded, who really cares. Heck you can even run log manifolds and make well over 300 crank-horsepower, and save another bundle by not buying headers. This 360 is practically begging you to build it. For another buncha money saved, just put the 2bbl set-up back on; you'll still make 300hp even on a bad day! Heck, put the 360 2bbl cam back in too! At 10.7 Scr you'll have so much torque, you can burn the tires off with 2.76 gears. Heck, those will get you to 60mph still in first gear, so you don't even have to shift!! lol.
Ok wait, I already did that;
except with a 340 bottom
and a 318 top and cam, lol.
In a 65 Valiant wagon. Sweetest combo I ever built.
Until my current combo that is................