According to my research, your 1996 Caddy Fleetwood has the GM 5.7L engine. The radiator adapter fitting you mentioned, part # D13018 would indeed be compatible with your vehicle, and could be used to replace the cracked fitting you mention.
Since the transmission line running into the radiator is a hard line, you will need to install a short length of 11/32 I.D. hose like # D13003 between the two fittings. You will also need some hose clamps, like those provided with # D13008 to secure... view full answer...