Thought I'd share a scenario that could be helpful. Replacing our 1990 6BTA transmission oil coolers, it's a bit of a struggle to get the plumbing right. The new coolers are the same size (almost) in length, but they are 1.5" tubes. The engine OEM is plumbed with a 1.5" raw water pump discharge, which goes into a silicone elbow reducing to 1.25". This attaches to a pipe, which is then attached by a short hose to the 1.25" OEM cooler. Replacement coolers that I found were all 1.5" pipe. I like Seaboard Marine so I use their coolers.
In researching our setup, I discovered that the silicone elbow on the pump can be used on the discharge end of the new cooler (1.5 to 1.25" pipe.) It even needs a 90 degree turn which fits perfectly.
Now for the intake side, I found a radiator hose that is sold under two numbers that has a short 90 on one end and plenty of hose to lop off the excess and do the ~12" run to the cooler. NAPA part #9683 or Gates part #23018. These 1.5" hoses fit from the pump to the cooler perfectly once the excess is trimmed off. On both of our engines we had to replace the shorter of the two hydraulic hoses, as it was 2" too short.
Hope this helps anyone going after this relatively easy job.