LA / Magnum blocks
I vote Magnum for all the same reasons
@Brooks James noted. I pulled a complete short block from a local junkyard for $150, couldn't find the mileage since it was a 2000 Dodge truck with the digital odometer and the wiring harness was hacked out. Based on the condition of the truck it was likely around 200k miles.
Bores still had crosshatch, all I did was install new rod bearings and 'sent it' with ported Edelbrocks, port-matched RPM intake and custom-ground hyd roller cam. Still running strong and tight after 6000 miles or so (including some track days and lots of fooling around) in my '70 Duster.
Superior pan and valve cover gasket - not worth much
better factory machining - debateable
Hyperuetetic pistons - no thank you, I've seen what they do when they fail
Metric rings - No much if any
Lighter rotating mass - Really? How much?
Heads as Good as any stock 340 - Not true
1.6 rocker arms
Factory roller camshaft
More compact, more overall engine compartment room - Not buying that one
Dead end bolt toles, not water leaking problems like LA - My early LA's have blind head bolts, never had any water leaks from threaded holes in the block
ETC
How many LA engines have you seen with 100k plus miles that have factory hone marks and almost no cylinder ridge Quite a few actually, used to get well maintained engines, lightly hone with torque plates and reuse the pistons on budget builds.
They do have better factory machining, I haven't confirmed that myself with measurements but I'd bet the decks are much straighter and the tolerances are much tighter than back in the 60s-70s. The casting quality of the block was visually much better than the handful of LA blocks I've inspected. I did check how far the pistons were down at TDC and they were all right at .050". LAs usually have more variation between cylinders. Pistons are also tighter-fitting in the bores than factory LAs.
The pistons may be hyper but I've never heard of one failing in a factory Magnum engine and they were installed in 1-ton pickups pulling 10k+ lbs and often had detonation issues due to the intake plenum gasket going bad and sucking in oil from the valley...
I don't know exactly how much lighter the rotating assembly is but it's enough to require offset balance weights on the damper and flywheel/flex plate about half of what a 360 LA requires.
Magnums keep the cylinder crosshatch even if they aren't maintained well and they keep it to 200k+ miles thanks to EFI. LAs won't do that.
I'd also say it's more of a wash regarding heads, J/X heads with 2.02 valves will have more peak flow but Magnums have more velocity and low-lift flow. But they're also "weaker" and prone to cracks between valve seats.
The rest I agree on. Externally the same as an LA, SBMs never had through-holes for the head bolts, and those better gaskets can be used on either LA or Magnum engines. LA valve covers work on Magnum heads btw I've done it.