I have a rebuilt 350 with no distributor, not sure how to set the timing?

There are 2 ways depending on how built the engine is. You can emove the timing cover and line the marks so the mark on the cam gear is at the bottom and the mark on the crank gear is at the top. When they line up together that is TDC.
Or you can remove the valve cover on the driver's side of the engine and turn the engine over until the timing mark on the bottom pulley come up on the 0 on the timing tab. Watch the valves cycle on #1 piston (front driver's side). As the pulley approaches 0 the intake should close and as you start past 0 the exhaust should start to open.That is TDC. If the valves don't operate in this exact manner then rotate the engine 1 full turn and then they should open and close properly.
Put the rotor button on the distributor and make sure that the vacuum advance is on the passenger's side of the engine before you try to put the distributor in. The rotor button should point at 3 o'clock because as you slide the distrbutor in it will turn some and should end up at 5 o'clock one it's seated. DO NOT hit it with a hammer or anything like that to seat it. You may have to turn or wiggle it and push down lightly to get it to fully seat on the engine but don't force it or you will break the bottom part that fits on the oil pump. If you have a long thin screwdriver you an turn the oil pump so that it lines up easier.
The firing order should be 1-8-4-3-6-5-7-2. Odd pistons on driver's side. Even on the passenger. They number front to back and the distributor turns clockwise. #1 is at 5 o'clock. Hope this helps.