318 Roller Build - Thoughts?

IMO, I'd skip the adjustable rocker arms and reuse the OE rockers and pushrods. The Lifter has .100 of travel and the first and last .010 are not good to use in actual use/calculations.

20mpg's may be a strech. I'm getting 17 on the Hwy. A MSD is being sought to help a tad. An overdrive would do it and add performance in it's deeper first gear and .69 overdriven gear.

3.55 X .69 = 2.4495 final drive gear ratio feel.
3.91 X .69 = 2.6979
4.10 X .69 = 2.829

Remember, the final drive ratio should work with your cam at hwy. speeds. Cam for this!

The 500 is the 904 with a overdrive
A 518 is the 727 with a over drive.

Though expensive for a on the cheap project, it is food for thought.
You'll also need to shorten your drive shaft and re-work the tranny mount. I know theres someone out there making a trans mount that bolts up and has a proper mount for it as well. A torque converter may be needed for this as well.

Again, not cheap, but IMO, a worth wile expense if your keeping the car and making lots of use with it.

I also do not think your cam to small. I'd rather see a slight increase to the duration, more like mine or a slight larger.