The coolant expansion tank caps used are as follows:
grey = 150 kPa (22 psi) superceded by the green cap
green = 150 kPa (22 psi) standard issue for all 700/900s
white = 100 kPa (15 psi) uncommon
black = 75 kPa (11 psi) standard issue for 140/240s
It's always best to use genuine Volvo or at least a reputable OEM brand.

The green caps provide for optimal engine operating conditions in 700/900s. Some people believe the lower pressure (240) black cap doesn't stress the plastic sides and fittings of the rad tank as much, especially in a hot turbo engine compartment and especially as the rad ages. With that thinking in mind I've decided the trade off is worth it and use a black cap in my turbo, but have kept the green cap in my NA.

On your rad problem, that is one ugly failure. A manufacturing flaw such as you describe may indeed be the root problem. On the other hand, a well-built rad may have been trying to hang on as long as possible until it finally blew under abnormal conditions. Although Nissens generally has a very good reputation, QC inspectors do have off days. Otherwise, it sure makes me think the pressure either rose rapidly and/or somehow got significantly higher than 22 psi and/or the rad had been under stress for a long period of time. The immediate suspect for on over-pressure condition is the rad cap and I'd automatically throw the old one out so that it can never be used again. The coincidence of this happened soon after water pump/t-stat replacement has to make you wonder, but this wouldn't be the first time a cooling system failure followed a pump replacement. With your experience I don't mean to insult you, but a few thoughts do come to mind. When you re-filled after changing the water pump, is there any chance it wasn't full? You often need to top up after a day or two. A rapid boil over (rise in pressure) could occur if there was a large air gap. As you note, there's no reason to suspect brutality with the upper coolant hose neck and that doesn't appear to have been the failure point in any case. Have you ever used a de-scaler or other additive? Loosened scale can partially plug a rad.

The last Nissens failure I had involved a slight weeping along that fingered pinch seem. I was topping up only every few months so I wasn't too concerned. The first real evidence was a tiny drop of fluid when I was rolling around under the car. For a 9+ year old rad it didn't owe me anything. I presume my leak would have gradually gotten worse and not blown as the problem started at the lower end, whereas in your case I agree it likely started in the middle as you suspect. Is there any evidence of a previous leak, such as coolant loss, or of leakage in that area, such as dried fluid stains?

So that raises the question, is there still something that's going to be stressing the new rad? Is the cooling fan able to run at both low and high speed? You can use OBD test mode 3 (see the OBD section in the 700/900 FAQ) to easily verify operation of both speeds. Presumably you've kept your rad flushes reasonably up to date, long-life coolant or otherwise. Rad fins full of bugs? Are you using the recommended thermostat? The owner's manual specs 198 deg for non-turbo and 189 deg for turbo. Again, the higher temps make for more optimal running conditions, but do hasten aging of underhood plastic components. If the correct temp isn't available in the aftermarket and you don't want to get one from Volvo then a slightly lower temp will suffice. If the dash gauge is normally dead horizontal then you're probably okay. Is the t-stat installed correctly? -legs down and weep hole up. Does the t-stat even have a weep hole? What are the odds the new t-stat has failed? I'd open up the t-stat housing and carefully give it a visual check and keep an eye on the temp gauge for a while. Do you operate in a hot climate? Tow a trailer? Have you offended the Volvo gods in any other way?
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now