The best thing to do is message @DELL-Cares as they are best suited to advice where to go from here.

Corsair Hydro Series H60 (2018) 

Don't forger the Y-Splitter adapter cables. The push/pull configuration with ML120 Pro fans has been done by many others . . . there is definitely room for the radiator sandwich in the cage. That being said, it will be a tight fit, so patience is important. Watch that Christmas video to see how it is done. I don't recall if it is specified in that video, but you will need four #6-32 bolts that are 30mm (1.25 inches) long to mount the fan/radiator to the top of the cage.

Take some photos of your project and share here, if you do not mind. It may be helpful to others trying a similar mod.

For a quick opinion benchmark, userbenchmark can be used: userbenchmark 

It's not a top of the line benchmark and sometimes is a bit skewed, but it is free and pretty quick, and gives you a baseline of some sorts.

Hey, the 3 pin versus 4 pin connector is simple: it can only go on 1 way, regardless of how many pins are used. If it does not slide onto the motherboard header easily simple flip it around and try again. Very little force should be used to slide this on.

The sata connector goes into any of the spare sata power plug connectors of the power supply. It DOES NOT go to the motherboard.

No problem. It's never a good idea to keep working hours on end trying to assemble components. Much better to take a break and try again tomorrow.

One other thing to keep in mind is that when you tighten down the pump assembly ensure to keep even pressure by tightening the mounting screws in a cross pattern, similar to how you would tighten a car rim. This will prevent the assembly from being installed under an angle that will in turn provide poor contact with the CPU surface. 

I always use my left hand to keep the pump firmly in place while tightening the screws in a cross pattern.

There should be additional SATA power cables stuffed up in the metal PSU box . . . but any SATA power connection should work. Connect the pump tach ( 1 wire ) cable to the FAN_CPU motherboard header to avoid the diagnostic startup error.

Looks like you did a great job! I think clearing CMOS is a good idea . . . but first;

the pump tack cable (1 wire ) connects to FAN_CPU mobo header

(click photo to embiggen)

Hi ProfessorW00d...... It's Worked, It's actually worked. You are a genius. There is absolutely no way I could have navigated through this upgrade without your brilliant help. I just can't believe that it started up and there were no error messages at all. I powered it down, then started it up again 5 times and every time it has started correctly. That is all down to you and you hard work supporting me. I cannot tell you how delighted and relieved I am. Thank you so very much for being so patient and for staying with me until the problem was fixed. We are leaving for a 4-day break in the morning (it's just gone midnight here) but I can leave in the knowledge that the PC is working. When I get back I will tidy up the loose cables inside and then produce a write up. I have taken lots of photos along the way and hopefully that will help others. All I can say now is a massive thank you. Take care and best wishes, Ed

Good to hear!! I enjoyed working through the project with you. Have a great family vacation!