Well, we just bought a new longblock 3.1 for my car, however we got a gasket set for a 2.8. We've had bad luck the whole way getting it and if they delivered us a 2.8 it wouldn't surprise us at all. How could I tell if it is a 2.8 or a 3.1 just from the longblock?

 

I think you'd need to measure the crank's throw; pistons should go "deeper" into the 3.1 since it's a stroker motor.

But I wouldn't know where to find that spec! If you had your old motor nearby you could do a comparision...