The aluminum radiator should be enough to cool the engine, but not so sure about the electric fans. I would go with a mechanical fan, preferably a factory type clutch fan, and you definitely need a shroud. Electric fans are nice, but running electrics without a mechanical or a shroud just won't cut it.

When in doubt, go back to the way the factory did it.

As for timing, that could also be your problem. You should just be able to get it in the ballpark without a timing light. Just raise the idle to about 1500 rpm, then turn the distributor (advance) until the engine starts to miss, then back it off a bit, and tighten it down. That will get you close enough.

There's also a chance your factory balancer may have slipped, a common problem with the Pontiac design. The outer ring slips on the inner hub, and trying to set the timing with a timing light is impossible. You could be 10, 15, even 40 degrees off using a timing light (if your balancer is new, or you have an aftermarket unit, ignore the info above).

No need to remove the coolant overflow, just remove the radiator cap, and take a look at the flow as described above.