The best way to fit the a sump so it won't leak is to coat both sides (block & sump flange) with a good coating of silicone sealant, then fit it to the block and then do up the retaining bolts with only your fingers. Leave it for a couple of days, then finish the job with a torque wrench. (Also make sure the sump flange is straight. It makes the job so much easier)

Is your sump plug is looking a bit dodgy because it's only ever been molested by a cheap shifting spanner? While it's out weld a large nut to the head of it so you've got a nice, large surface to grip next time. The same applies for the gearbox and diff sump plugs.

You may have to do this to the sump plug while it is on the car. Some Fords with alloy sumps are famous in the trade for difficult to remove sump plugs. It often takes this sort of action to allow an oil change to take place. (It happened to me on my RS2000 - Peter).