6 lug to 5 lug

[Big70]

So I have a guy selling these axles I wanna know if they will fit into 70 C10 12 bolt. He told me all I need to worry about is measuring from back of axel housing where the four bolts hold on backing plate to the lip kind of close to the pumpkin... he said there were two different size axels 21” 1/4 and 20”?.?.? Need help cause if I can use these axels I want to jump on them. I’m planning on taking the truck to 5 lug. I do get 21 1/4 to the inside lip closes to the pumpkin. Any help is good help!! Thank you in advance!!!