In reference to your concern with front sight and lug being close to one another, there is a possible approach:
tin BOTH areas for your soldering ahead of time, both the lug and the sight locations. Tin means flow solder on the steel to coat it. heat and flux, while rubbing the barrel areas with a little piece of steel wool spreads the solder -oh-so-nicely.
Heat ONLY enough to melt the solder. Too much heat will cook the flux, and you'll need to start over again.
Then tin both the front sight and the lug. All you need is a thin coat.
1) solder the lug on first, as it is probably the bigger of the heat requirement. With both parts tinned, you may not need to add solder. Clean off all flux. As long as there is no flux, the solder is not going to re-flow.
2) clean front sight location, and re-flux. solder the sight on with as little heat as possible. Remove heat once the tinned parts 'suck down'.