OK, so double flare it is.

Is the copper tube OK, or should I get brass?

use steel fuel/brake line, not copper or brass.

Brass unions and steel line sound good.
Make sure to get a little extra steel line to practice the double flare on. All the flaring tools are pretty much the same (hard to work with and line up) untill you get into the $200.
If you can find a place that rents a good flaring tool. I would do that. I know i came across a few online.

Alright, so after all this talk, I've finally decided I'm going to try to do what Mike suggested a number of posts ago...namely, mod an L67 fuel rail with fittings and hard line so that it'* 3/8" inlet and 5/16" outlet hook up in the same general area where the L36 fuel lines would hook up to the L36 fuel rail.

I decided to do this because of some potential fitment issues:
1) Modding the L36 fuel rail would force me to cut out a piece of the EGR heat shield and would put the rear rail too close to--perhaps even touching--the EGR valve metal mounting pedestal, which could heat the fuel in the rail dangerously.
2) I would have to come up with some kind of convoluted attachment to get the FR mounting foot near the alternator to fit the L67 LIM FR stud.
3) The rear rail fit might not fit under the L67 alternator bracketing I'm going to need to get for the topswap.
4) The front rail might get in the way of the BCS/BBV.

One thing I don't know...are the inlet and outlet ends of the L36 vs. L67 fuel rail identical?
(That is, ignoring that they're in the wrong places, would the fuel line quick disconnect fittings on my L36 snap onto the L67 fuel rail?)

Because if they're not, I'll have to buy one of each to hack apart.

I'm almost positive the L36 and L67 fuel rail inlet and outlets are the same. I can check if no one else chimes in..

I'm almost positive the L36 and L67 fuel rail inlet and outlets are the same. I can check if no one else chimes in..