First, take that 2-carb setup and pitch it off a bridge into a very deep river so you won't be tempted to try to fish it out again. Inline sixes either need one carburetor or three. Next- - - -go rob a bank. To make the head support the kind of modifications you're suggesting takes money- - - -by the pound. Research "lump port" modification of the cylinder head before you even think about doing any porting or valve size increase- - - -those huge head bolt columns in the middle of the intake ports needs to go away.
Now- - - -do a complete bottom end rebuild before you think about cam, carb, or intake/exhaust work. While you're in there, install a set of 307 Chevy V8 pistons- - - -they're flat tops instead of having the 1/4" dish top of the 250 piston, and you're going to need the extra compression to support any sort of a lumpy cam. The last hotrod 250 I built got a .050" milled head and flat top pistons, and nothing crashed. However, I didn't get stupid on camshaft timing and lift. Anything over about a 270 degree duration cam has no business on the street. If you want an idle lope, take one spark plug wire off. You're going to need a strong foundation to build on, and putting a lot of money into the upper end of an engine in less than brand new condition makes no sense at all.
"It costs money to go fast- - - - -how fast can you afford to go?"
I'm in the process of accumulating the bits and pieces to build a 230 with 3 Harley-Davidson side draft carbs and enough intake/exhaust and bottom end modifications for it to live comfortably at over 7K RPM. I'm expecting to spend somewhere upwards of $5,000.00 before I hear it run, and I'll be doing almost all of my own machine work. The NASCAR tool steel connecting rods alone I'll be using for that engine sell new for over $2,000.00. I'll be using reconditioned "one race" rods from a place that salvages retired round track engines, for a considerably lower cost.
Jerry
