Mystery overheating

[BAsque1]

Ok guys I am very much stumped with this situation:

I purchased a new Stainless steel surge tank for my 427 S/O and got it installed. The previous set up was as follows: thermostat 180o the engine's optimum operating temp has always been 180-195 if it reaches 200o the electric fans kick on beautifully and temp regresses to normal operating temperature, cap on the old tank is 16 lbs.

The new tank was installed and the problems began, I changed the thermostat to a 160o just to replace with a fresh thermostat, installed the tank without any problems or leaks. Sunday driving back from a show was a very hot day and the fans did not kick on at 200o as normally will do and by the time I got home and [placed the car in the garage, the bottom hose loosened up from the radiator and lost the coolant. Next day I reconnected the hose, no leaks all is fine.

Turned the car on the fans don't kick on at 200o just for fun and giggles I replaced the new thermostat with the old 180o and replace the radiator cap with the old 16lbs cap so the set up was exact as the original. Turned the car on temp went over 200o fans failed to go on, check the fuses of the fans and the relays and all is fine, I happen to place the test light on the on line sending unit located in the return hose and the fans kicked on this confirms to me the fans are fine the fuses are fine and the relays are fine.

Frustrated with this situation and to avoid possible overheating I by passed the online temperature switch with a manual switch so I can control the fans at 200o from the cockpit. Today I tested this set up and the temp is also high up to 250o immediately I turned the car off and the fans on at home.
If all is the same what the hell is going on? I am still using Prestone 50-50 same as before, the water pump is circulating there are no leaks what could I have missed?
Any thoughts are very much welcome please.
Thanks
Lou (BASQUE1)