Good gravy---that is
filthy! How and why did that happen?
Before beginning I'd remove all seats, the front factory floor covering, the foot well pieces, park on a nose-up attitude and hose that sucker out.
The rear A/C & heating unit should also be thoroughly cleaned inside and out, using compressed air if possible. I might be tempted to remove some cover screws and see how fouled the coils are---they're probably pretty dirty too.
Good luck on your build---hope to see some progress when this thread makes it into the Homebuilt thread!