Well . . . IT WORKED!!
Even despite me messing up the final number a bit! I used a piece of aluminum angle, a piece of aluminum bar and 3 clamps. The angle piece helped me keep things in a straight line and the stamp shaft 90 degrees to the frame and straight on. The bar helped me keep it horizontally oriented and from moving with repeated hammerings to get the number/letter to show up enough. I moved the bar along the angle the width of a stamp shaft each time that I finished a digit or letter, so that they would be evenly spaced. I carefully laid out the stamps in the correct order and orientation and checked it multiple times throughout the entire process to ensure I got it correctly matching.
The Ficha Technica said it was located on the right frame member. The challenge came in that there was little room to swing a hammer anywhere to make the stamps very deep. So, I had to do it on the side of the frame just under the passenger seat where I know it appears on UK vehicles. That was the only place I had just enough room to swing, and it was marginal. But I was able to get the digits at least deep enough to hold some paint. Unfortunately, the jig moved on me before I realized it on the final digit. I was able to recover, but barely. It just looked like I messed it up with the scraper trying to find the numbers.
I scraped right down to bare metal before starting which turned out the best anyway due the shallowness of the depressions. That also gave me the excuse to say why we couldn't find it the first time (due to buildup of paint, undercoating and dirt). Then I sprayed black paint over the numbers, waited about 5 min for it to partially dry, then wiped off the excess with a paper towel dampened with solvent. The result looked somewhat convincing.
At least it was convincing enough I guess because
I passed the ITV! (. . . and that's why they call me "Moto-MacGyver")
If anyone here is crazy enough to want to try to do what I did, importing a N.A. vehicle to Spain, I can give you lots of tips and info!
I hope you all found this post entertaining if nothing else.
P.S. - If anyone is wondering why I brought my truck to Spain, Rangers here are
extremely expensive here and mine still looks almost new, with very little rust on the frame and none on the body. It purrs like a kitten and has a number of upgrades and custom things done to it, like a removable winch for winching motorcycles into the back, a custom fitted hard tonneau cover, stainless steel MBRP exhaust, etc. The cost of replacing it here and re-customizing would have cost me far more than the $6k Cdn I paid for the truck originally plus my additions. Although it has over 350,000km on it, runs perfectly and gets tons of envious looks here. I saw one old guy the other day who was parked beside me taking photos of it.

Now the challenge will come for getting parts and doing repairs, but 90% of that I will be doing myself and I was aware of the challenges and risks.