Spare tire and rim size for 70 Duster

Hi folks.

I lucked out yesterday and found a spare tire cardboard cover piece from a 69 4 door valiant for my 70 Duster. It fits pretty nicely.

The problem is that the spare that came with ny Duster is a 'normal' tire/rim width(14 inches). The tire sits about 3 inches too high in the depression in the trunk for the cardboard to be down flat. I don't want to brake the ends off of it either due to it being too high.

The junked valiant also had a rotten spare tire in the trunk with the nice new looking cardboard. Do I need to go back and get this rim and put a thinner tire on it for the spare or can I just use the rim I have and put a narrower tire on?

Thanks for any info.

The Big E