I'm a little bit different because I'm based in Australia and these are rare here either way. But I say dump in it what ever your comfortable with to get the result you want. Mines a 440, that has an RT rear trim/lights (cos I love the look!) other wise debadged. I've gone for a Day 2 look (or maybe a period pro-tour?). I don't hide that its not an RT, but I've been asked maybe twice about it? most people just go 'fwoooaaarr, sweet charger man!' lol.
Yup a number or a name will get you more money than a base even tho they are the same thing (lets be honest here), and if your doing it for cash then you have the wrong car (spec and model). But if your doing it to build a 'sweet car man', then you've got the right car. Nothing makes me smile like stomping the go-fast-pedal and hearing that lumpy fire breather roar!
My advise? your budget is your initial on road budget. Blow it on things that CANNOT be easily changed later -IE body work (including clearancing for trans or modern engines etc), then must haves (seat belts etc), then its things that are a PITA to change - glass (and mech's for them), brakes, interior, drive train - kind of in that order.
This will be my second summer driving mine and I'm still tweaking things. couple of things to go on the interior (dash trim is all really), thinking about a borgeson, front bumper could do with a change, GV overdrive potentially then diff ratio to suit...
Then forget what you spent on it (so you don't get depressed! lol) and go for a drive instead and bask in the 'fwooooaaars'!