First thing you need to know is what condition the weights and pins are in on the advance. If they are worn out you probably better off to find a different distributor. A 300 has 22 degree advance. If weights are ok, I would just fill in the limiting hole in the plate and do some bending on the spring ends. Get them so springs keep it fully retarded and then the fun begins.
You need to mark the pulley for 22 degrees before top dead center and with a timing light keep playing with the length of the hole and tension on springs until you get a nice gradual advance from idle speed to full 22 degree advance at 1450 rpm.
I measure the hole and do some math as to how much to fill it in for starting point. Liquid steel works fine to fill with. Takes time, saves money.