Help me build a 318!

So here is what I am doing, I need a decent gas mileage 318 for the Super Coupe for my cross country (WA-NY and back) road trip next year...

Here is what I have:

'71 318 running and driving in the Swinger right now (OD 4spd and 3.55SG 8 3/4)
Bottom end is stock right now, it has rebuilt 302 heads, a holly 650 4150 (was a 2bbl motor) Edelbrock performer and a set of hooker long tube headers. I think I am getting about 18mpg but I am going to double check that tomorrow.

I have a spread bore Holly 650 DP I am planning on putting on to see if I can bring the millage up a little bit maybe 20mpg?

Now it's got a bit of power but as you all know a stock 318 isn't anything spectacular.

So here is what I am thinking. It needs rings because it's blowing a little right now (a lot less with the head change) so take it to the machine shop and have it hot tanked, checked out and honed, balance a set of '68 318 rods and pistons to give me a lighter rotating assembly and little more compression, add a windage tray, timing chain tensioner, and double roller timing chain and bolt it all back together with new rings bearings and gaskets.
As for cam I was thinking something similar to a stock 340 cam. (Even a used 340 cam would be fine with me, 68 4spd cam being preferred)

Then install in the Super Coupe with the OD 4spd and a 3.55SG 8 1/4.

So how well do you all think this combo would work?
What kind of gas mileage would I be able to pull down?
Should I just rering and bearing the engine to try to keep decent MPG for my cross country roadtrip?