It is not unusual to have packing pieces between the hull and the mast rail. I prefer Delrin that I cut from one of those cheap kitchens cutting boards. The holes in the mast and in the tabernacle should line up. More details here.
Before you do that check if the hull did not sink in. Some older boats do not have enough strength to support the mast whit the full rig tension on. If the GRP below the mast is dented or cracked in any way you should consider adding a wooden support inside the forward tank. There are some good essays on the Danish site that will help. Go here: Cracks in mast seat 1. Cracks in mast seat 2. (I use Google-translate to translate those web pages to English)
Finally, exchange your the Aluminium pin for a 6 mm stainless one. A 6 mm pin (minimum size allowed while racing) allows a bit more play while trimming. Alternatively use a 6 mm galvanised steel bolt and wing nut and replace when it starts to rust. I bet you can replace it six times for the price of one SS mast pin.