Are there vertical ribs on the sides of the block? If so it would be a service replacement block and would not have casting numbers but would have a date code. It's kind of hard to see the date code and being the engine is covered in crap does not make it easier. The date code can best be seen if the block is upside down and unless it's been sand blasted also should be there. The casting number should be there also but again the crap on the engine is probably making it difficult to see. That's if it's not a service block. Service blocks were just over the counter replacement blocks and fine to use. The heads are probably C8-H as these were pretty common and found on lots of 390's. $100 is a fair price as you really don't know what your getting being the engine is frozen and all. You can at least re sell the heads for $200 or so if the engine is pretty trashed. Even if it's a 352 block at a 4.00 bore you can open it up the 4.05 390 no problem but the crank will have a 3.50 stroke. Figure the block will need to be bored and cleaned up. The crank at least polished if not turned. Rods re bushed possibly and the heads reconditioned. Pistons might be salvageable as well. How did the engine freeze up would be my next question?? Was it running and just poorly stored and rusted or did it seize up while running? As far as the Mustangs go this is what I know. Unless the 67 was a big block car the shock towers might need to be cut for the 390 to fit. The 68 will be fine as Ford changed all the Mustangs to the big block reinforced style shock towers. As for the 69 -70 I'm pretty sure the 390 will fit but not positive if the shock towers were different for the small block and big block cars. I'm talking FE series big block not the 385 (429/460) series. I don't think a 429 will drop in easily but like anything can be done. G.