Thanks both of you guys. As an amateur I couldn't translate the imbalance stats but I think I'll agree that if they mean the 360 is the most imbalanced of the applicable V-8s, then out of the box it probably will be what's needed for that engine. Why make it heavier than the heaviest requirement of the bunch? The plate must be made to work as is on something without modification.
I just got back from the machine shop. Very nice guy and, typical for his profession, enjoyed brainstorming this problem (machinists who don't like thinking outside the box usually don't last long in that business). He can handle the drilling project okay but the downside is if 360 doesn't work out, it would be difficult going back to a 318 because finding another one of those small 122 tooth flywheels to fit the bell housing might be dicey. So I'm not cancelling the adaptor plate order (forty bucks with shipping is no big deal even if it doesn't work out). He referred me to a Mopar high performance specialist in town before we dig into this further. He also showed me his balancing machine. Very nice and very new. Downside is to use it would require me completely tearing down the engine as he not only needs the crank but also a connecting rod which would mean pulling the head, which means pulling the marine manifold, probably new rings for at least one cylinder, etc., etc. I just as well rebuild the engine (though I'm sure it's almost like new). Problem there is the marine engines used different parts than same vehicle engines (e.g. 160 tooth flywheel in 360 is marine specific). For one thing, as I understand it the oil ports in bearings are larger. Might be tough finding the right parts to do a rebuild.
The arrow on top of 360 engine says rotation is to right as I'm looking at it from front. Is that the same as vehicle rotation? Starter is on the "driver's side" (right side as I look at the engine from the front). Okay, I looked on line ... starter is on same side as truck bell housing so must be same rotation.