25mm adapters will work. You could even use wider adapters since the wheels are so small. 15mm or 20mm is not required for such a narrow wheel, so go with 25mm. You would want the adapters to have a lip on the front that is the correct outside diameter to match the center bore of the wheels you're using. A good set of custom adapters costs $300+ for a set of 4.

The Volvo center bore is 65.1mm. The depth of the hub area which is 65.1mm is 19mm deep on the 240 front hub. So the centerbore will need to be approximately 20mm deep into the spacer to ensure that it sits flat on the hub. With a 25mm adapter, this would still allow a smaller diameter lip on the front to suit the wheels in question.

www.wheeladapter.com can make them for you, but they're not cheap. $85 a piece with the lip on the front. He uses M12 studs, which should work fine on Nissan wheels. 1/2" studs use 3/4" socket on the lugnuts. M12 uses 19mm. It's basically the same, so don't worry about different size sockets. 3/4" = 19.05mm.
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