We will not use a tank that has the filler in the bed floor so use only side fill tanks. The early Blazer tanks have a shape that fits in the factory spare tire position and require zero frame trimming. A fill tube is run under bed floor out through the side and then up to a motorcycle pop up gas cap in the left rear fender. As the photos show the tank DOES NOT hang low and has dual exhaust with no clearance issues.
That looks like a solid option for the 3100/3200s. When you say "early" blazer, approximately what year range are you talking about?
