Advise on adding a sump to stock tank

I want to add a sump to my stock tank in the 66 Dart.Once I have it out and drained of gas,whats the best fluid to use to wash it out to get all the gas and fumes out before welding?Anybodyelse done this?Any advise/help would be appreaciated as I don,t want to blow/burn myself up.LOL Thanks.

I could always take it to a chassis shop,but trying to save $$ before Christmas.