I'll toss in my $0.02, perhaps they'll help:
1) Cranking... yours cranked very weak, and made lots of pinion gear noise... you may have a problem with that pinion, the ring gear, or starter bushings... AND... since it hit as soon as you released the rod, I suspect that your starter draw was impacting ignition voltage, thus, coil current.
One of the notes guys included, was the keyswitch situation- it is NOT unusual for a keyswitch IGNITION contact in a starter switch to fail in the 'START" position... where it will crank fine, but not 'hit' 'till you release the key. As Charlie noted, magnetos don't have this problem. Your WD, if stock (pull rod start) would not exhibit this issue either, because it doesn't have a RUN vs START keyswitch to fail.
What CAN happen, though... is the battery GROUND cable can have a weak enough connection to result in high resistance, which during cranking, means you will NOT have sufficient coil current available to fire the plugs. It doesn't matter if the ground and battery cables look perfect, and were brand-new yesterday, they, and their connections, can exhibit resistance. Furthermore, they're fine-stranded core constitutes a 'capillary tube'... moisture will wick from the ends of that wire, right up the cable, and oxidize the copper while leaving NO indication at either end of any issue. I've found them by moving from one end to the other, inch by inch, bending them a bit, and feeing for 'crunchy' type responses. I have, in all sorts of applications, found copper wire turned green in the middle of a perfectly beautiful looking run. The best way to seek out this problem, is to put a known-good booster cable from the starter to the battery... if it cranks faster, your ground path is compromised!
2nd... the popping... and the sparking... More than likely that you have a leaky exhaust manifold gasket, and perhaps some erosion of the casting around where exhaust leaks occur. Pulling the manifold and cleaning the area up, assessing and mitigating it's erosion, reinstalling will make it much happier. Until that is done, however, you will likely NOT have much success dialing in the tuning, as an exhaust leak there will cause lots of deviation in cylinder-to-cylinder vacuum readings, and suspicious-looking compression readings.
Another thing... with it sitting a long time, SOME of the valve springs sat compressed, while others not so much. Now, Allis used VERY VERY GOOD spring material, so they didn't 'sack out' as much as other engines would, but it's possible that you've got a few that are a smidgen weak. As others noted, you probably have a little crud hiding in there from years of sitting and prior consequences. One attempt at treatment is to loosen up the valve lash adjustment a few thousandths more, and run it a little bit (make it work), then reset the lash to stock. This will frequently knock crud loose from the stem and around the seats.
Another thing that will cause backfiring, is a worn out distributor shaft or point cam. These distributors DO experience distributor shaft bushing wear, and when they do, the timing becomes unstable as the shaft wobbles around in the diz body. Pop the distributor cap off, yank the rotor, and hold the shaft between your thumb and first finger, and attempt to wiggle it side-to-side, in every direction. If you find slop, it's worn, and you'll have erratic timing as a result.
There are advance weights in the distributor base. Clean them up good.
Make sure the oil-bath air filter isn't plugged up... and that it has OIL rather than water, in the bottom. ;-) Tell the wasps to go away, nix their nest from the stack. When you have the manifold off, rod it out good, just in case some of them wasps built a tiny-house in the ports.
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