It's not just the alternator output that will "be enough". It's also the reserve of the battery and the health of your whole charging system- especially the wiring- double especially if you're not sure of the condition of your bulkhead connector, dash harness, ammeter, and engine harness.
I have all of that in good shape, and a trunk mounted group 31 truck battery for PLENTY of reserve. I have no problems. My ammeter (new autometer) hardly budges when I flick the switch. About the same if the headlights are on.

As far as trimming, I'd say you can take about .5 inch off evenly around the housing without the fan hitting the core. That is up to you. With my radiator package, I needed about 4.5 inches.
True hot rodding at play here!