I'm in need of some simple explanations of the external balance situation with the 360 LA engine...

I have one diagram which shows the three 1 13/64" holes, but it's not really clear enough when it comes to the locations for the holes. And getting that size drill is difficult, expensive and probably fraught with difficulty if you get the depth of the holes just a little wrong.

The existing flywheel has about six or seven holes in it, smaller in diameter (about 3/4") and on a different radius. More than that, these holes are round at the bottom, not pointed as they'd be with a drill. This flywheel was prepared for me nine years ago in Spokane, it seems to have been good enough, I've driven the van over 30,000 miles since then, but the A833 isn't really up to the job (needing a lower first gear) so I'm fitting an NV4500 and that requires the flywheel used with the 11" clutch, that is the 143-tooth flywheel.

Is it possible to find a decent diagram for the three 1 13/64" holes? Are there any acknowledged alternatives like the pattern drilled for me in Spokane? Are there any alternatives of another kind?

See if you can still buy a new 11.0 inch flywheel for a 360 powered truck from your local Dodge dealer
I've seen those flywheels with those big holes on the starter side of the flywheels more than once

This is a job best left to a machine shop to do, unless you have the tools to properly locate those holes.

Since you neglect to mention what van it is, I made an assumption and looked at rockauto.

LUK LFW219
PERFECTION CLUTCH 503413

Both new, both under $90.00, plus shipping and handling, both 143 tooth, both with the 360 imbalance holes.

If you are insistent in doing it yourself the drawing you want is probably in the MoParts tech archive. If not the old MP books have it or you could just google it like I did.