Rip it off and replace.

I'd remove the molding, install the flooring and re-install or replace the original molding so it overlaps the flooring with a clean, square edge.

Why, if you are installing 3/4 inch hardwood flooring that is nailed down along the groove of every board, would you need a 1/2" "expansion gap"? If the board closest to the wall gets moved that far by expansion,it's going to pull the nails loose.

The 1/2" expansion gap is manufacturers specs. It seems to be a grey area with ranges of 1/4 to 3/4 inch from what I read.

We just removed the baseboard from our 1975 house using a prybar and a utility knife. We plan to reuse it so were careful and it was not difficult. Utility knife across the top to cut through caulk, prybar carefully inserted along behind and slowly work our way along. Our new floor is going to require a 5/8 expansion gap, it will be floating elastilon installation, but our baseboard is short and narrow (2.25 x .5, I think) so will not cover the gap alone and I haven't found any trim thick enough on its own, so am looking at "beefing up" the trim with some ancillary pieces like a square block and quarter round or something.

New molding is surprisingly expensive, if you are doing a large area, so that is one motivator for us to reuse, plus we don't want to waste something that still is workable.

I think you may need a larger expansion gap than you think, see this link:

Well, GW thinks that site is spam. I googled expansion gap solid hardwood and came across it pretty easily. It explains that solid wood typically expands more from the tongue side, and the recommended gap varies by species and cut (rift, quarter, etc) and that solid 3/4 is one of the most expansive types of flooring.

I should mention we are also thinking about under-cutting the sheetrock. That is another way to gain room for the expansion gap.