390 issues

Howdy fellas from sunny washington (yeah right)
so i bought this country sedan wagon fro a guy who wanted to go fast down the strip, kind of going about it the wrong way but who am i to critisize...
anyways its got a new 390 with dual exhaust and a *270 cam. a bit extreme in my eyes which is why i want to change it out.
i dont have a manual but i have a good understanding of how to do it. maybe you guys can help me out with some questions...
i believe he has the old 352 dizzy in it from the old tired engine that was replaced. is this possible? can a 390 run with a 352 dizz? the dist. i have currently has pertronix kit in it as well.
i wont mind a stock 390 cam but is there a small step up from stock but not crazy like a 270!? i read on another post that 270+ is high performance so thats a bit much for me.
i believe to replace the cam i have to remove the carb intake, lifters, accessories from the front and slide her our??
the wagon is my daily driver and family cruzer / trip taker. any help with the tired old fella would be appreciated.
the problems that i have been trying to iron out with the 390 has been constant chattering when i go shut the engine off. and ive been all over the place with the timing nothing seems to cure it. the only thing that even comes close to not chattering is turning the idle screws waaay down on the edelbrock carb to hardly nothing. which in turns makes it idle like garbage!
well any help with "henry" as my little boy calls it (i think he heard henry ford somewheres and read the tailgate reading ford so thats what he calls it) would be appreciated it.
$arge-