The backfires are a sure indication of a timing problem. First thing I'd check is that the plug wires are in the proper position on the cap - that's an easy check, and a not uncommon mistake after all the wires have been pulled.

If that's not the problem, I'd try a purely mechanical timing procedure.

Pull the spark plugs, take off the valve cover and open up the timing cover. First, set the crank to the cam. Set the tensioner to loosen the belt, put a dowel in the #1 spark plug hole, then turn the crankshaft until the dowel rises to its highest point, and the crank timing marks are lined up. Then turn the cam so that after the intake valve on #1 opens and closes, both valves on #1 and #4 are closed. Check the position of the cam marks, and adjust the cam until they meet.

Finally, turn the intermediate shaft until the rotor points to #1 - the i-shaft marks should line up. Release the tensioner to fix the positions, making sure that none of the gears move.

Then turn the crank slowly , and visually confirm that the valves are opening and closing at the right time - e.g, that the intake on #1 opens when the dowel in the #1 hole is descending, and that the #1 exhaust valve opens on the proper upward stroke of the piston. Switch the dowel to the #4 hole, make sure of the same sequence.

Put it all back together, making sure that the firing order is set up accurately.

It's kind of a tedious procedure, but using it confirms that the timing is correct.