The Gen 1's often leaked at the joint of the crossover for the thermostat where it connected to the cylinder heads. Later cars had an improved gasket.
The real problem was that one of the holes for the crossover attaching bolts was slightly out of location laterally. You could start the bolts, but the mating surface to the cylinder head would be slightly off when the crossover was torqued down. The solution is to enlarge the hole. You can also use a higher torque on the bolts.
Again, without any gasket in place, seat the crossover and try to start the bolts. you will see that one won't line up.