After a spate of P0442s (EVAP small leak), I took my '04 XC70 to a shop for a smoke pressure test.

A hairline crack has developed where one hose bib (fuel filler vent hose) attaches to the tank body.

Was this originally a plastic heat weld? What would be the best repair option?

Keep in mind, the repair must be done in situ -- reaching in from the side thru the rear suspension trusses.
This car has AWD....too much work to drop the tank. Please compare some options....fuel-resistant epoxy?....melt plastic surface with soldering iron?....hot glue gun??? It would help to know what is the plastic material I'm trying to repair.

Under normal operation (vented fuel tank), there is no oozing of gasoline. It oozed some drips during the pressure test. So, it's a tiny crack. Enough to be picked up by EVAP test.

If it is the vent hose, small plastic tube, any fuel resistant epoxy should work.

Here is a diagram, it shows two parts to the vent hose:

https://www.volvopartswebstore.com/showAssembly.aspx?ukey_assembly=1180197&ukey_make=865&ukey_model=20832&modelYear=2002&ukey_category=7072&ukey_driveLine=472&ukey_trimLevel=28254
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Keeping it running is better than buying new

I tried a fuel-compatible epoxy (Perma-Chem), but it failed to adhere.

No less than 3 Volvo shops advised me that the fuel tank would have to be replaced, estimates around $3200 (the Haldex and propeller shaft have to be removed, as well as the rear seats, fuel pump and level sender).
The new tank assy. part was quoted at $1400.

I was able to fix the leak using plastic welding.

I went to the junkyard, and sawed out a 6"x 6" (150 x 150 mm) piece of fuel tank from the same MY, thus assuring plastics compatibility.

Since the leak was along a circular joint around the filler vent hose bib, I cut a donut shaped patch out of the scrap material. I used a 2" (49mm) dia. hole saw to cut a circular disk, then used a 13/16" (21mm) dia. boring bit to cut out the donut hole. I judged the patch to be too thick to melt, so in a vise I sawed it into two donuts half as thick, and only used one. The patch was about 2mm thick.

The car's rear end was jacked up about 6", and the pass. side wheel removed. This is so that I could address the repair from both the side and underneath.

At Harbor Freight, I bought the Chicago Electric 80W plastic welding iron ($16). The tip is just the right size, but I advise getting some practice with scrap material. It's low enough power, there is little risk of burning through the tank (1/4" thick fiberglass-reinforced high-density polyethylene).

The hose must be removed from the bib to get the donut patch on. I used a Dremel cutter to release the factory crimp-style hose clamp. It's safer to do the welding with the hose put back on the bib (closing off gasoline fumes).

The welding was a bit messy. The hose bib was likely a different kind of plastic as the tank, and didn't blend as easily into the weld.

That said, the Check Engine Light faults have stopped (with tank between 15-85% full), and so the plastic weld did the trick.

(I'll have to pay the Volvo dealer $95 to clear the SRS "urgent service needed" code. I drove the car with the rear seats removed to the shop to get a smoke-pressure test - you have to pin down a hairline leak this way. Only the Dealer-enabled VIDA can reset SRS faults, and it leaves a record attached to the VIN at Volvo Corp.
SOmething to think about before removing the seat(s)).

I like your repair! Shaving the donut in half must have taken a while.

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Keeping it running is better than buying new

It was pretty straightforward. The full thickness donut was 6 mm. I put it in a vise. I used a wood miter hand saw, slicing thru 90 degrees at a time, staying on the midplane of the donut. I got two patch piece halves about 2 mm thick. They don't have to be sanded....ragged edges melt just fine.