Hello and welcome!
Colin is a great source for axles and anything else you may need.
I use Michelin LTX M/S2 P235/75/15XL tires on my '72 Ambassador. They have four years on them now and this summer I'll probably be replacing them with the newer Michelin Defender LTX M/S 235/75/15XL tires. Another good option for your Sovereign
would be GoodYear Wrangler H/T LT235/75/15 Load Range C tires.
Make sure your original rims are in good condition with a detailed visual inspection.