Clutch size - What fits, what doesn't.

In to process of mating a 5.3 to a SM465 trans. I have ordered the sachs flywheel NFW1050, which is for a later model 4.8 or 6.0 engine that also had a manual transmission. This is to make up for the missing .400 of an inch in the newer engine.

I've yet to recieve it, however I'm looking at clutch options ahead of time. The clutch I run now behind my sbc is a stock, 12" clutch that came on any half ton or heavier pickup truck from the 80's, running the same transmission. I would like to purchase a new clutch of the same size since this truck does a lot of towing, offroading etc, and I feel as though a 12" is designed for just that.

From what I've read from conversions, everyone else runs an 11" clutch. While this is an option, I'd hate to be barreling down the highway, having the clutch slip out on hills because its to small, or spend a small fortune on a high-performance 11" clutch to match what a stock replacement 12" would do.

Now hear me out, because I don't have any parts to compare. But since a 12" clutch came on a 163 tooth flywheel behind a sbc, would it not bolt up to the new sachs flywheel, which is also 163 toothed? Or would the bolt pattern on the flywheel be smaller for some reason? Is the pressure plate of a 12" and 11" the same, with just the clutch size being different, or are they perfectly matched to their specific size?