Hi David!
It takes x things to make a reciprocating piston engine run:
Fuel
Air
Compression
Source of ignition
Proper timing of all events.
You've obviously got compression and spark. If it's popping, I'll assume that you've got fuel and air coming INTO the combustion chamber. From there, it all comes down to timing.
If you rebuilt the engine properly, I'll assume that your valve-lash settings are correct, and that your camshaft timing mark was installed in proper synch with crankshaft. I don't remember wether your 70 used the Automatic Compression Release feature or the Centrifugal Spark Advance, but these devices MUST be right in order for it to kick-off well.
Ignition timing: throw away everything you know about setting point gap, and use a continuity-tester and the flywheel marks to set the initial timing... OR, if you're comfortable twiddling, you can open or close the point gap setting a tad and see if it'll fire-and-go, after which you'll need to adjust point gap to make it run 'right'.
Oh... and just because you've got spark at the plug when it's OUT of the cylinder (in free air) doesn't mean you'll have proper spark when it's IN the cylinder and under compression pressure. Takes much more electron-whomp to jump that little gap when there's compression. Re-check your plug wire, and don't be afraid to swap out the coil, condenser, or points to eliminate possible points-of-problem. Realize that a bad condenser can break an otherwise fine ignition system, and just a little oil or crud on the points can cause marginal success. make sure your grounds are all good, too!
Ask yourself- Was the engine running okay when you took it down for a rebuild? If so, the ignition and carbeuration should've been fine... but the carb might've been re-adjusted for a poor-running engine...
DK