Will, OK I see where you are coming from. If it were me I wouldn't do it but it's your locomotive and your hobby and I don't judge. Just do whatever it is that makes you happy.
The reason I gave my opinion as to sell it earlier is because many model railroaders often change their interests over time myself included. They change scales, eras, leave the hobby, move to a new home, come back to the hobby, etc and while you say you will never sell the engine now you never know what life is going to throw at you in the future. You know what they say about the best laid plans.
I have one other suggestion and I know this suggestion is so insane I probably should be shot or banned from the forum for even thinking it but why not just build a loop of 2 rail track to run this locomotive on? If it is as nice as you say and I am sure it is, then wouldn't it look better on 2 rail track as compared to Fastrack? Remember PS3 doesn't care how many rails are underneath it.
I am sorry but I disagree with this statement: "Anyone can buy a Lionel locomotive and slap a couple extra details on it, but this is something special."
I agree with you that if you are successful with this conversion it will be something special, however it isn't easy to detail die-cast locomotives. It's much easier to add details to brass. If you have read threads by forum member Wowak who has detailed and kitbashed 3 rail die cast locomotives I'm sure you wouldn't say he just slaps the stuff on there. What he does is also something special. Both are.