Unless you can find the original books, the original owner with hard evidence such as photos and license papers or the original dealership (if still in existence) has records practically impossible. Trust your gut instinct. Ask the current owners what they know about its history and WRITE IT DOWN. You will be amazed at what you forget if you dont.

I am no expert but I would imagine putting GTS bits into a HQ Ute is a relatively new happening, say the last 10-15 years, before that HQ's were not very popular, hence the use as a shooting ute. You are lucky to find it in a condition even worth considering restoration. I am not saying it didn't happen (putting GTS bits on Utes), it just wouldn't have been common.