need advice on 20" tires

[NicoSlam]

I purchased a set of boss 315 20" (8.5") with 305/50/20 tires. I tried to fit on my truck but the tire was obviously too big for the front. The rear fit with 1.25" spacers but I need help on selecting the front tires. What is the narrowest tire I can put on an 8.5" rim? I see many threads where 255/40/20seem to be common but can I go narrower. Once I select the width I can play with the aspect ratio to match the rear. I have a 1969 with a 4/6 drop. Pics of trucks with 20"s on 1967-70 with a 4/6 drop would be great.