Funny that you mention a 218 @ 0.050 cam for the 318, since the camshaft I'd use If I were to build the small block would be a comp cams 268H.......
I actually started out by picking the 3.21 gears. My car has 26 inch tall tires. Cruise RPMS at 65mph calc out to 2700, and 2300 at 55mph. Going to 3.55 gears would have put my my 65mph rpms at 3000. I know some guys have no problems cruising at 3k rpms, but I would rather cruise below that. Like you said, settling for 3.21 gears puts a limit in my camshaft selection. Although I would still choose the same cam even if I had an overdrive trans and 4.10 gears.
Next thing I did was calc out the compression ratio. Using KB flat tops 0.012" down the hole, 0.028" thick gaskets and eddy heads with 63cc chambers calcs out to 9.5. Milling the heads to 60cc brings it up to 9.9. With a compression ratio in this range, I picked out he comp 268H for 2 reasons. First, in
this thread they dynoed a 9.1:1 318 with eddy heads and 252h camshaft. A build like that is my idea of a "max effort daily driver". Cranking pressure on that engine was measured at 170psi, and dynamic compression calcs out at 7.88:1 using the wallace calculator. I would have no problem using a camshaft that size, since it worked so good in that build. But using that camshaft on a 9.5:1 engine results in a 8.22:1 dcr. I feel that's a bit too high. The 268 with the same static compression ratio spits out a 7.81:1 dynamic comp ratio. Second reason I chose this cam is because I expect it to work with my factory converter. Someone with way more experience can chime in on this, but I understand this cam will come on somewhere between 1500 and 2000 rpms. And regarding David Vizard's 128 rule, a 110 LSA is what it calls for in this application (318 with 2.02 valves and between 9.5-10.5 comp).