I have a TH400 in my Monte and I ran an external cooler as well. Your fitting sizes sound right from what I remember running on mine but I have a couple suggestions for you. First, heat is the biggest enemy of trannys, right? And the more fluid you can flow through it the cooler it will be..along with an external cooler too. Well, I went several steps ahead and installed a two-qt. aluminum TCI pan, then ran my lines from the tranny to an external remote filter (using a K&N) block from Perma-Cool with a threaded port for a temp. gauge, then into my cooler w/fan and back to the tranny again. This way, I gained a total of about 4 additional quarts of fluid to the system capacity to help fight off the extreme heat. I did not care for any aftermarket electric fan to use for mounting on my cooler nor did I care much for the integrated fans that are sold with the coolers either. I used a primary engine cooling fan (the steel-cased one-9" in diameter) from an Eagle Talon and it mated almost perfectly to the Perma-Cool cooler I used. The fan itself moves 2100 cfm @ 19 amps and my temps never go over 130 degrees. Just thought I'd share what I've done with great success. -Jabin