Can you use existing floor joists space for air flow when installing new furnace in home after it has been finished? Wooden joists 14 and half by 10 and sealed from bottom with plywood and silicone. Also was going to paint inside of space with latex semi gloss paint. No metal duct work at all straight run approx 25 feet long. Thanks

Why would you want to do that. Easier to put in duct work.

Some places do not allow the joist spaces to be used because of fire spread. In addition, older houses with T&G sub floor can leak air. As Harold said, lot easier to just pull flex duct.

Short answer: NO
Longer answer: Some jurisdictions may allow panned off floor joists to be used as returns but none of which I am aware will allow it for supply duct.