I'm bringing back this old post because I just had the same issue.
The spring clip on the replacement axle was noticably larger than the OE. I swapped them out
I completed the job with a sledge-hammer... not joking. I screwed the axle nut onto the axle then tapped the end gently with an 8lb sledge palmed in my hand. I made sure the axle was supported and straight before tapping. You need the inertia from a sledge to move the heavy axle into position, you dont want to hit the axle hard enough ruin something. A small hammer would likely ruin the axle end without seating the axle. Just my $.02