35 years in an Illinois barn?
So, just how bad is the rust?
Seriously, all Panteras need a thorough and knowledgeable rust inspection. They will rust through on so many body panels as to make any restoration unwise if paying a shop for the labor.
This one really needs inspection. Also, 35 years of high humidity storage can surface pit all the ZF internals not sitting in gear oil to the point of destroying a LOT (most) of the expensive pieces.
Clean, well-presented Panteras are starting to see a noticeable increase in their selling prices. But basket cases like yours present unique attention-needed areas that keep many quick-buck car-flippers out of the picture.
Your market is wanna-be owners (wise and not so much) and current owners.
With all that in mind I'd think you might currently find a sight-unseen, hungry but not too wise buyer willing to offer $30K-$40K
A quick sale, sight unseen (rust ) to a self-professed "knowledgeable" buyer might see you get $10K-$15K.
Personally, sight unseen as it now is, I wouldn't offer more than $2,500.
Condition is what will drive the car's value.
Leave it as an unknown-condition barn find and prepare to potentially lose thousands unless you smoke out one of those not so wise buyers.
You can and should do a full chassis, body and doors,coolant,brake system rust inspection. That is a lot of inspecting.
If you are not prepared to pull the ZF and open it for selling photos realize a useless ZF will set the new owner back at least $7K, and that is on a very good day. Everyone knows the ZF is a high dollar piece. But not if rusted.
If after a full inspection you can offer a rust free chassis and ZF to a potential buyer, I'd think you'd get an easy $30K from many buyers, and perhaps $40K depending on paint and interior condition.
Do you want to maximize your profit or minimize the selling task?
You can't have both.