If you are looking for a Pinto engine then the p100 might be a suitable donor.
I doubt if any of the rear axle components will be of any use to the Roadster.
The rear drums and shoes will most likely be bigger than the standard Sierra drums.
One drawback is the postage costs of the heavy rear end parts, look out for parts that are local to you.
I have seen quite a few complete Sierras at my local breakers since I started my build. I have never taken any parts off them for Ebay because of the high postage costs.
A pity really. It would have been nice to earn a few pennies for my build & perhaps help fellow builders out a little.