Very frustrating rear end dilema

[DoubleL]

So I bought a posi rearend from a 1969 C-10. I thought it would be a bolt in for my 66 c20 eaton rear end. I have someone putting it in for me but found out it needs at least an offset u-joint and an adjustable control arm for the mod which would be several hundred dollars. He is really advising me against dropping down to a smaller 6-lug axle. I am starting to second guess it as well.
My first choice would be to rebuild this rearend but with no aftermarket parts for this particular rear end I am looking at close to $1400 to rebuild it.
Of course I would have to eventually convert my front to 6-lug as well if I went with the back end I have.
To complicate matters more i have already bought new tires and rims for the 6-lug...... really confused right now...