okay i have a dodge B300 van with a 318 in it with has a 2barrel carb that needs a rebuild . my goal is get the best mpg as possible and may a little bit more power from my van .
Now i wonder whit which car and cam i should go to reach my goal , i thought about a 500cfm edelbrock carb but have no idea which cam would be the best for me , or should i even stay with my 2 barrel carb and just change my cam and rebuild the old carb?
if i go with 4 barell i would go with a edelbrock intake for low rpms.
What do you would do in my case?

Lock up 727 or overdrive.

SP2P Eddie intake, or Offy Dual Port, stock iron spreadbore intake, Eddie Performer.

Summit RV grind, I have a 252H-10 Comp cam in my B200 with an Offy DP.

Thermoquad or Mopar Quadrajet carb.

Flat top pistons. 302 heads with a MPG R/V type port and polish job.

i dont know which tranny i have , i guess it is a regular 727 3 speed no overdrive or lockup. the heads i like to leave stock, i am more concerned if i shoud stay with the two barrel or if a 4 barrel is better for mpg?

That must be some sight in Switzerland!
I won't say the smarty stuff like chop and channel.
Build a good tight engine that performs well in the RPM range you use the van in.
Do all the other usual stuff, like air up the tires, keep the weight down (remove unneeded stuff inside), and get it aligned.
There was an article many years ago where they did wind testing of a van using different aftermarket parts. Front spoiler, side skirts, rake (lowered front), rear spoiler and rear wing (the kind that diverts air on to the back windows).
IIRC, the best was a level ride height and the front spoiler. The side skirts were a wash, unless they extended so far down they were unsuitable on the street, and anything on the rear made it worse.

i dont plan any changes i only like to get a bit better mpg via carb , cam or/and intake

A 300 van will have some heavy duty axles-hubs suspension etc, I don't know if it will ever get good mpg, only decent highway mpg maybe. And I have never found improvement on mpg going from the tiny 2 bbl to a 4 bbl. I'd rebuild the carb, check timing etc and see how it does.

A small (600cfm or less) 4-bbl and 4-bbl intake manifold will give you slighty better mpg than a 2-bbl, all else being equal.

As suggested, use a mild cam which gives good torque in your desired rpm range, and a low restriction exhaust system is also a good idea.

Please see my signature line for the link to the Mopar B-van forum, too.