[bpatton]

From personal experience you don't want to do that.... Making flares for 410a is a sort of art form and they have a high probability of leaking due to the high pressures involved.
I did the same thing you want to do with a mini split and now I'm going to have to go back and braze the lineset so I can have some peace of mind.
If you are going to braze, nitrogen is a MUST. You have to flow some nitrogen through the lines so that carbon doesn't build up inside of the lineset with the high temperatures. You can braze with a map gas torch if you have it, just use a stick of Sil-Fos for the braze and you should be good.
The best advice I have on flares is to make sure to deburr the inside of the copper before you flare it and then use a dab of Nylog flare sealer on the seat of the flare and on the threads too.

Typically in the HVAC world they dont use "unions" like they do in plumbing, Its a male flare stub that's brazed onto one end or the other of the line set. You can get brass plumbing unions at hardware stores that have male ends on both sides but that increases the possibility of leaks when there is now four flares in that joint instead of just two (one per line)