[padresag]

I would just shift where you fell comfortable as the situation is. it will be quite fine to shft it into third at 20 mph unless you have a load in the rear or that you might be going up a hill. on my 1t with a 250 i usually just shift into 4th between 15 and 20 mph. there really is no need to take it to 4,000 rpm unless you want to hear it scream
Chevrolet I6 engs have always pulled nicely from lower rpms
ron