I would extend the evap line up (rather than vent lines down) - and tie it all together above the new tank. I ran into the same kind of issue when I injected the 57 - although no evap - the in-tank pump "spill off" (its a returnless style EFI) was splashing the vent line and after a hard run it would puke gasoline all over the top of the tank and leave dme to quarter sixzed spots on the ground - simply raising the end of the vent to 12" above the tank by going into the fender well (was at only 4) fixed it.
The next issue you will have is when (if) you extend the filler. You will have to run a vent line with check valve to the filler neck, as close to the cap as you can get it - from your existing vent system or else you will force fuel into the current vent\evap line every time you fill up. Another lesson the 57 taught me - and it only has a three inch neck (bed mounted deck fill with tank underbed between rails). Just as the gas nozzle would click off (or level came into neck if filling with jerry bottle) about 1/4 gallon would come out of the vent at high pressure and all over the rear axle (with 15 gallon tank at 4.3mpg locked into high octane - fueling is something I do often with this beast so we aren't just talking pennies hitting the ground!)... just tying the neck to the vent stopped it. I used a bulkhead fitting carefully placed in the neoprene hose that connect the tank neck to the deck cap neck and ran it to a tee on the vent fitting on the pump/sender flange with a regular check valve between the neck and tank (allowing flow to neck) and the rollover check mounted under the bed with the vent line running up into the fender well with a filter on the end - yea I know not a good place for it as the well is very watered and dusty - but this is a fair weather and street only machine.
The only real difference (functionally) between your system and an atmospheric vent like my old slug is the evap catches and holds vapors until an operating parameter of the engine (usually idle and decel) can sacrifice vacuum to slurp it into the engine. In my case fuel hits the ground - in yours it plugs and locks the evap!
J Wm Bishop EA, ASADE
The wagon should, of course, be as light as possible, but strength should not be sacrificed to lightness, for on any but the regularly traveled roads, the wagon will get many a
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