A search of this forum probably would have turned this up as I've posted it a few times before.
You can buy the inline independent check valves for the 7xx at FCP:
http://www.fcpgroton.com/product-exec/product_id/5150/nm/Volvo_740_8_Valve_Fuel_Pump_Check_Valve_1989_1992_Bosch_/category_id/149
It looks like the exact same part was used on the 9xx until MY 1994.
It will take some creative rigging because the fuel lines on your 850 are vinyl fuel lines and the check valve will need flare nut connections on metal line. It can definitely be done though.
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1998 V70 AWD->FWD->AWD Turbo 215k+
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