Simon,
I'm close to the same point in my sbf build. According to the information I have, you definatley want to install the
oil pump driveshaft from below.
The Tinnerman retainer (washer) you refer to is important because it will (if properly installed) prevent the pump driveshaft from being lifted out of the
oil pump when removing the distributor. If you don't use the retainer, the pump driveshaft could end up in your
oil pan if you ever remove the distributor. I think it would be a lot more bother to have to pull the sump to retrieve the pump driveshaft after you already have the motor in the car.
Regards,
Greg