Weld to the adapter, not your factory hitch. Theres a reason that hitch has an odd size, the Volvo factory hitches in europe are rated for roughly 5000 lbs instead of the 3500 that the american hitches sized for the vehicle and that the DOT says it can pull, this goes for older models as well. They sell frame stifners to up the load capacity even more on some models. but here in the US we are stuck with the wimpy stuff.
If you have to cheap out and not get the factory insert, just weld up slats on a cheap insert, but dont ruin that nice hitch :-)
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-------Robert,:'00 XC70. '95 960, '93 940t, '90 240 wagon, '94 940SE, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually