[Gromit]

I've read - (I believe on this site) - that others have vent it back to the driver's side upper tank vent. But this would require the California version of the 1970; so you would need to order one of those replacement Evaporative Emission Control "EEC" type tanks - but at least those tanks seem to be readily available.

Here is a photo of the vent I'm referring to - it is the first year of the EEC and it was in California vehicle starting in 1970.

The I.D. of the vents is 5/16" and the Fuel Supply lines are same as non-EEC 3/8"

I am not saying that this is optimum or that I am recommending it but just that you might consider checking further to see who has done it that way. If you had that 1970 EEC tank you may also need to install vent lines and a charcoal canister if you want all the hookups to be as stock.