OK....here's what I did on my '70.
The factory vent system was rusted out and when I filled the tank, the gasoline would spill out onto the ground.
I bought a rollover vent and some hose from Summit Racing
https://www.summitracing.com/parts/TNK-VVR
I made a bracket about 10-12 inches long from a piece of 1/8" x 1" wide aluminum.
I bent both ends to 90° and drilled a hole for the vent to attach and the other end for a bolt to fasten it to the floor.
I attached the vent to the bracket, the hose to the vent, zip tied it to the bracket, and put the assembly up in between the inner and outer bedsides and bolted it thru the floor.
Then I attached the other end to a "Y" block I made from a piece of brass and 1/4" NPT x 5/16" hose barb fittings.
On the other side of the Y block I attached the hoses to the existing tank vents...mine has two.
I made the Y block in my machine shop but you may be able to use a store bought "T" fitting.
Here's some pictures after it was installed.
this is the bottom of the bracket bolted thru the floor where the old vent was
this is the two lines going to the tank vents and the Y block
this is the line running from the Y block over and up to the rollover vent.
