Well the first thing to do is get the casting number from the flange the transmission bolts to. I think its on the passenger side. Then same flange on the driver's side there is a casting # starting with a letter with 2 or 3 numbers. Example: C28. On the front passenger side right below where the head is bolted, there is a flange with the engine ID numbers unless the block has been milled. You can either post them here or look them up on this wesite. Chevy Casting Number identification - Block casting numbers
BTW the heads also have casting numbers under the valve covers.