First, I LOVE your truck!
Second... "don't do it!!!! You'll regret it!!!!"
I'd probably try asking for the price you want WITH the trans first. As you said, you're going to have to find a guy who wants a truck just like that. Pro-street with patina is a small niche. And the guy who sees your vision would likely like a manual and appreciate the M22. If you swap to auto, you WILL open up the market since lots of people like an auto for a "cruiser" pickup and more and more people have never even had a manual transmission....
I think 10-12 would be finding that "right person"
Judging from craigslist, ebay, and sales here....it's likely a 6-8K truck right in the overall market now mainly because of the current economy...too many deals out there...I've seen trucks that have easily 25K in parts in them selling for 10-12K on ebay and craigslist.