Fire it up! Make sure you have the trans full so the pump bushing isn't dry and you may want to have it in neutral (727's don't pump in park). Also, if there is no exhaust on it, you may want to call the police first, before the neighbors do. LOL
Starting a new engine without mufflers is fun but you can't hear if the engine is making any funny/bad noises during break in. I always try and hook up something, even short pipes with clamp on mufflers just to keep the noise level down so I can hear the engine (rockers, flywheel/flexplate scraping) or any other strange sounds.