If you are using your stock '72 spindles that small spindle bearing seal may not be used unless you got a very late '72 unit. It is typically on 73+. That entire kit you have in the picture is technically for late '72 and up but will work on the earlier 72 if you don't use that small seal. The factory 72 seal set was a different animal with a large press in metal seal and a brass thrust washer. In late '72 they went to the plastic thrust washer with that large rubber v seal. The spindle is also then machined a little deeper and the spindle bearing is pressed deeper so that that small v seal seats within the end of the spindle against the bearing. Not using that small v seal is not a big deal unless your doing a lot of water running through creeks, etc.
From 72 to 73 the Chevy Dana 44 had several small changes including axle u joint size, slinger size and seal kit, spindle and seal kit.
Hard to capture it all in this email but I went through all of this a few months ago when I put new Spicer front axle shafts in. You can only get 73+ now. The 71/72 are no longer available so applications show them all as 71 and up regardless of all the above small changes.
Quad4x4 website is a good source of information if you read the parts notes as well as Bill of Material information found on Dana Spicer website.
I linked here a Quad 4x4 page on their website. Scroll down to the "Spindle Bearing and Seals" section.
http://www.quad4x4.com/gm44front1968-1972.html
As far as seating the snap ring on the end of the axle as long as you have assembled correctly and set the outer hub bearing pre-load with the double nuts and slot washer with the pin correctly, you should be good. As others have mentioned some slight prying outboard on the axle shaft at the u-joint will give you the space you need to seat it. I needed a friend to help. I found it took more than 2 hands to do it all at once. Those locking hubs don't look factory to me but I doubt that has anything to do with the snap ring if it was together before.