1969 Dart 340 cam selection help !!!

You will need to have your machinist check the deck height because it might already be decked and if it's shorter than 9.600 you wil lwant to consider the piston choices carefully.
Operating under the assumption it's factory which is usually somewhere around 9.630:
Pistons - I'd use KB hypers - KB243s. They are cheap, light, & strong. You will want to deck the block down to 9.610-9.600. This will yeild a static compression around 10.3-10.5:1. You will also want him to gap the top ring per KB's specifications.
Rods - Eagle SIR rods are cheap and fine for your application - in addition to being very light. Have them cycled and the big ends checked afterwords to make sure they're good.
Cam - If he wants hydraulic roller - stay much smaller. I would be looking at the XR274HR-10. You will need to replace the valve springs that come on the RPM heads with the ones that Comp recommends - that's fairly normal. Also watch the lifetrs you run. There's a post about it and you might want to run Howards or Hughes lifters on it...
That would be my recipe following the direction you asked for. Personally - a hydraulic flat tappet would be just as good for less money and the same maintenance & I don't read this post as "I want to run low 12s" or "I need a 450hp 340".

Edit - assuming the RPM heads are the closed chamber LA versions and you're going to have them checked and the valve job fixed by a competant shop. If he says they're fine out of the box without having them apart on his bench - I'd go somewhere else.