Yes you can have a positive pressure booth by just blowing air in, you do so by adjusting the exhaust outlet size.
Unless you're willing to invest a LOT of money in a UL listed hazardous location fan unit, placing your air mover outside the booth, blowing in, is your only safe option. Putting a filter in front of a box fan will NOT make it safe to position it in the exhaust stream.
Accumulation of overspray buildup is another problem, you'll need a filter over your exhaust intake, and to clean the duct frequently. Accumulation of combustible, dried overspray is another very good reason you can't put a box fan in the exhaust stream.
In addition to the fan, you should have no electric fixture or light, radio, switch, or other source of ignition in the booth. Solvent vapors do separate from paint fog, so there is no way to determine if you're getting close to explosive limits by sight. Given the size of booth you propose, I'd say there's a good chance you could easily achieve hazardous levels of concentration from both a health and fire standpoint.
If you'll consider a carburetor's function, and compare that with what you're doing shooting lacquer in a spray booth, you can to begin to appreciate the hazard level. Except you don't stand inside an engine's cylinder.
Finally, from a quality standpoint, no matter how you heat/cool/dehumidify your booth, that conditioned, optimal air is going to be (literally) exhausted pretty quickly. Conditioned make-up air is a major consideration in a painting operation.