Siezed 273 v/8

I have just bought a 1964 valiant siget 200 273 v/8 pushbutton auto .The car was stored inside for about 15 years . IT was running when it was parked .But it will not turn over . We have tried putting diesel down the spark plug holes and waited for a few days but we could not get it to move. There are many products on the market like engine release but at $20.00 per kit, per cylinder, plus $5.00 per kit for shipping . Looking for any info on this product or any other ideas that members may have used to try and unsieze a motor. Any help would be welcomed. Just trying to get it running without tearing it apart. Would like to try to keep stock. Thanks in advance for your time and knowledge, Thanks Valiant64