I'm thinking of putting together a 7.62x39 build with the the extra/currently unused lower receiver I have sitting in my closet. (already have a 5.56/.223 build, and looking for something different)

Is it recommended to use a billet upper receiver on such a build?

I see the pre-assembled uppers kits for sale on the net, but most of those appear to be using non-billet upper receivers. (at least from what I can see)

Billet is not needed.

Ok, that's what I needed to know.

Thanks!

Only reason it is necessary to use the billet uppers so far as I know is if you are going to run a 24"+ barrel as the weight up there can flex the receiver, which just can't be good.  Doubt anyone is going to run a 7.62x39 barrel that long though, so yer good with the stock upper.

Ok. That's where I was confused. I understood billet = more weight. I was thinking that there may be some sort of requirement for the extra mass due to the caliber being larger than 5.56/.223, and potential chamber pressure differences. (more powder in the cartridge, depending on the particular load.)

I think I'll spend the $$ differences on aspects that will have a better ROI.

The piece of paper/bad guy/varmint the rifle is pointed at won't care about the aesthetics.  

If you are going to use AK mags on a MGI / AR47 lower you might want to reconsider that billet upper.  
Because you need to clearence the upper for AK mags and the forged ones will end up  thin after doing so.  Ones which start with more meat, will end up with more meat left.  


This is probably why you heard using the billet ones, or a thicker upper like the DPMS lo pro in the first place.  Because they are thicker.

If you are going to use C-Products mags, which have become unobtanium in 30rd configuration for less than $30 each.
It doesn't matter.

This.

I have a spare "standard" lower receiver on hand, and was looking to build something other than another 5.56/.223.

Doing a 7.62x39 build with it seems to be an interesting twist. I just wanted to make sure I start off on the right path with the parts list.

Does not require anything special just because it is 7.62X39 in terms of the upper blank material.
The bolt/extractors have been known to have a shorter life when compared to .223, but not the upper in general.