1974-1976 were nice becuase of the flat top knuckles, but used the small bearing spindle.
big bearing vs small only becomes an issue if you're looking to upgrade to the 8-lug spindles, which require the larger bearing. I know plenty who run the small bearing D44's and I've never heard of a spindle breaking.

All 3/4 ton axles after mid-'78 used internal locking hubs retained by locking rings.
don't think this is always true. I had a '79 3/4-ton D44 that used external hubs.

After 1980 all 3/4 ton chevy front axles were 10 bolt...?
i don't know that there was a 3/4-ton 10-bolt. I think all 3/4-ton's were D44s.

I am planning on running 37/38's which is just about the limit of the Dana 44 with no modifications.
well, this is about the limit of a D44, *with* modifications. you won't run a stock one reliably at this size, you'll need CTMs and alloy shafts to really secure it.

When searching for an axle online is there anyway that I can determine the difference between a truck with a 44 and one with a 10 bolt?
visually, the two look very different. the insides are different as well I believe, spline count, ring gear diameter, etc. Also, i'm pretty sure Chevy only used D44s in the 70's and went to the 10-bolt in the 80's.

Which dana 44's had 6 lug and which had 8 lugs?
1/2-ton = 6-lug, 3/4-ton = 8-lug

Can I simply swap in flat top knuckles that I find on eBay to a Dana 44? I know that almost all chevy axles have the driver side flat top.
yes. *all* 4wd chevy's had a flat top drivers side due to their unique steering design. Chevy driver's side will be milled and drilled and ready to go, but all passenger side flat tops will need work before they're ready to bolt an arm to.

-Rob