The timing tape mentioned is the cheapest solution, but I could never get it to stick for long.
A 69' 351w balancer is correct for a passenger side water pump. It will have a 3 bolt balancer pulley. In 70, Ford moved the water pump timing marks to the passenger side on most vehicles.
You could also use a pre 70' 289/302 balancer.
I got my early balancer from Summit--Pioneer #872008. If you are using the early pulleys, you should make sure that they match the balancer. Some have "inny" center hubs that should match the balancer. Summit carries them to under a slightly different part #. Summit also carries balancers with timing marks in multiple locations and made for multiple pulley types, kind of a one balancer fits all solution. Just make sure that what every you get has 28oz imbalance.