3 cyls at +.030 and 1 cyl @ +.040 Whatdoyouthink?

what could possibly go wrong??

"in the day" the Datsun guys all had #2 cylinder at the limit of spec bores and strokes, and the rest... well, let's just say a std breath-down could tell the difference quite easily.
It worked for them on a rather massive size difference, I can't see it not working on a small difference.
Or you could always wrap a .020 piston with aluminum foil and JB weld.

he pissed in his what?

With the 3-bearing main, I am a little worried about flexing crankshaft and breaking it. I KNOW something is going to break--- (that's more or less the point, no?) ---it's just how long will it last before it breaks and is there anything I should consider doing so I can maximize potential driver time. I'm not under a time pressure at the moment, so there's time to maybe do it slightly less wrong.

For starters, I'm thinking I need to make the #1 hole the +.040 (if I can actually have a choice) because it's right by the balancer pulley and furthest from the flywheel (that hangs 6-8 inches or so behind the rear main).

HAHAHA you care about a hundredth of an inch. In a LeMon. Seriously, it will run better than 75% of the cars out there if you have access to any measuring device more advanced than a ruler.

Just run it.   Worry more about rod length and any damage from previous failures. 

If the motor had actually been balanced before I'd go through and set rod weights and match piston weights but this is Lemons and if they are close it is probably good enough.

Checking the new piston against one of the others is a good idea, if it has a different deck height or weight it may vibrate a bit more or knock on only that cylinder ( if it has more height ).

If it has more deck height you can sand a bit of dish into it to match the CC of the chambers so you have the same compression across all 4.

Yeah, I knew that actually. Also the top couple lands are usually a couple .000x smaller than the barrel due to extra expansion at the crown too. I was thinking chuck it up in the lathe then see how close to round it is. I think others are probably right though that the engine won't know the difference if you have one .040 and the rest .030.... Since you are modding anyway, why not carve some skirt away and balance them to reduce drag? The skirt is just to prevent piston rocking, so is only really needed 90 degrees to the pin - I bet those old pistons have lots of room for improvement.  Did you know ring drag is like 20% of engine friction? I've been tempted to ditch the 2nd compression ring and see what happens (or get low tension rings if they are available)... would probably burn oil, but would run cooler and convert more of the power into forward motion... all kinds of stuff that would be fun to try in Lemons world that I would never do on one of my "good" cars.

Also do remember to use slightly higher ring clearance, at the high end of the acceptable range, on the top ring than normal street rings call for as road racing with a bad radiator and worn out water pump gets the top ring quite hot.

A preliminary check on a highly un-scientific scale shows that the +.040 piston is over 1 ounce lighter than the other 3 pistons of +.030.

and the adjacent piston to the broken one had a broken top ring. And despite putting the ring gaps  120-180degrees out from one another, the oil rings (not the spring spacer) all aligned -- on each piston, and also gaps on rings  2&3 also aligned or nearly so. The top rings were 180 out from the others, though.

Bearings look good (for used bearings). Crank looks ok, too. Just #4 rod journal has some scores, which it had when it went back together originally.

The main caps are grooved sorta-- like the mains spun a bearing but they never did. wonder if it's from the caps or bearings shuffling under continuous revs, or something like that. The outer edges of the main bearings show different wear/coloring than the middle of the bearings too.

Well, you and I both know that it only hits about 5300-5500 rpms max. I guess that it could be too many revs, but my money is on it all being too out of balance.