You shouldnt re-use the same gasket, specially for the ridiculous cost of it.
If you want to clean the inside nice and clean, let it sink in gasoline but in a closed container so the vapors dont go!
Nice to see people undertake projects and take care of their rides! Did you notice Haynes tells you to unplug the battery before you do ANYTHING on the car??
If you want to clean the engine bay nicely, use some "SIMPLE GREEN" but mix it with some warm water, spray it all down, use a kitchen brush and scrub that sucker! All the fugly gunk will flex and you'll be happy as simple green did 70% of the werk
good luck, and keep us posted!