Hmm since the tank was empty, rust could certainly be an issue that will clog filters in no time and give you endless headaches since fuel blockages always seem like something else. If you plan to leave a tank for a while, its always best to leave it full since the fuel is oil based it keeps it nice and clean.

Yeah get the tank out and give it a good knock about with nuts as roconnor said. Flush with some clean fuel and then use a lighter to look inside for anything left.. On second thoughts no dont do that, using a torch would be a better idea :) Rinse and reapeat until it looks all good, but Dont be too violent with the shaking since you dont want to damage the tank.

I can appreciate what its like to work for what seems like slave labour rates as I am sure most tradies here can vouch for. Lucky its a datsun.