Can I put an '04 headlight on my '00 without any electrical/connections/mounting problems???

If you're talking about the new style headlight (glass only), I changed my '02 headlight over to the new style in '04.

I liked the bar & shield on the lamp. It also gives a more focused light.

No surprises, it changed out easily.
Used the same connector.

Well, I'm looking to put the new Nacelle and I'm not sure what I'll need. So, I thought if I could find one cheap enough, I'd get it just in case.

If you're talking about the new style headlight (glass only), I changed my '02 headlight over to the new style in '04.

I liked the bar & shield on the lamp. It also gives a more focused light.

No surprises, it changed out easily.
Used the same connector.

Actually, the reflector optics headlamp was new for '05.

Maybe I'm a year off. Been buying too many parts and losing track of when I did it.
I do like that new headlamp.

I beleive the 2004 & 2000 lamps are the same.

I suspect you are refering to the 2005 refelctor optic light, p/n is 68297-05.

Fits '00-later XL, Dyna™, FX Softail®, and '05 VRSCB models equipped with 5-3/4" headlamp.'00-'04 Dyna™ (except FXDWG) models require adapter ring P/N 67883-04.