Solid axle D44 from an Early Bronco is your best bet...
If you're not goin to be wheelin it really hard a D 30 from the same will work for you as well.
Your existing coil buckets can be made to hold the top of the springs, your radius arm brackets will need to be moved back on the frame ( I think one of the needed holes is already there, just need to drill the other ones.)
You will need the diff, radius arms, lower coil buckets and coils from the EB... match up the gear ratios when you buy and you are good to go.
Buy a drum brake diff and save some cash, the disc brake conversion is easy to do to either EB diff, and there will be some brake hose conversions you will need to ponder as well.