LS Swap in 4x4 71 longbed

[JDO AK]

I just bought my second C10 and this one is for my son. I'm planning a LS swap and would like to know what kind of suggestions people have on a few things

Looking for better gas mileage
Looking for reliability

I was thinking a 5.3 or 4.8 with a 4L60 is this a good choice?

Also will a T-case from a earliy 2000 motor tranny 4x4 work as well as long as I use a manual shift?

I wanted to change the 4:10 rear end to 3:73 posi any options either way?

Thanks,

Justin