If you don't have a workstand hang your bike by it's saddle so the rear wheel is in the air. While turning the crank with your hand, gently try to shift into the big/big. If it won't go, you need a longer chain.
If it was my bike, I'd skip the test. To me, new freewheel = fresh chain. Then I size the new chain using the big/big method.
Thanks, the derailleur doesn't seem overly stressed at all in big/big gears (looked at sheldon brown and a couple other sites). I'll just keep the original chain, the beauty of this bike is that it has essentially never been ridden, so the chain is like brand new. The original owner told me he only rode it once, it's seems to be true as the braking surface on the rims along with everything else looks unused. He probably didn't ride it because he lives in some hills and the bike is geared so high, he said he liked riding his mountain bike better.