I'd be worried about the quality of cheap O-rings too.... as much of a PIA as it is to do the job it's just not worth the risk to me. And if they do leak you'll be spending a ton of money on oil and coolant and trying to flush the lube and cooling systems. Been there, done that.... not fun.

Also, putting the new rings in requires care and patience. Put the O-rings on the tube bundle, not in the end caps. One of the service manuals out there tells you to put the small O-rings in the end caps and then try to assemble it... it doesn't work. Learned that one the hard way. Putting both on the bundle seems to work well and make sure to use plenty of lube too.

I've also seen a hydro jib made up for these using a chunk on steel drilled for the bolts and ports and an air fitting on one of the ports. You bolt the new cooler on, fill one side of it with water and pressurize the other with air... Leaks will show as bubbles. I can't remember the exact details, but it seemed pretty easy to set up and for sure worth it.