Alan,
This German site might help. It gives dimensions for the 8-UN series, all of which are 8TPI, and might well be what you've got. Machinery's Handbook describes 8-UN as 'a uniform pitch series for large diameters. Although originally intended for high-pressure-joint bolts and nuts it is now widely used as a substitute for the Coarse-Thread Series for diameters larger than 1 inch." It's a valid Unified size, just not bog-standard.
For 1½" 8TPI the German table suggests a 35.1mm tap drill, which is about right for cutting an 8tpi internal thread to a depth of 0.06766" (Source: Machinery's)
When tackling confusing threads, I usually do a trial run on an Aluminium test piece first. Checking the result against the other thread will show if you've got the dimensions about right or need to think again.
The Unified spec says the internal thread should be truncated (flat) at the bottom rather than cut to the full sharp depth of 0.10825". It should also be rounded at the top. Beyond blunting the sharp top of external threads with a fine file I don't worry too much about either detail. If male and female mate I'm happy.
Making a new thread, I usually cut to a little less than theoretical, test for fit, and then take a succession of smaller cuts until the thread engages smoothly.
The annoying this about this sort of job is I usually mess up the first attempt in a muck sweat of time wasting anxiety, then make a poor second attempt that fits. If I go to the trouble of making a third, it will be made in half the time, be closer to size and have a much better finish. Make four and it becomes almost trivially easy to churn out good results…
Dave