Replacing the engine would be easier than engine-and-transmission. Still time consuming work, though. The big part is hoisting the old one out (not to mention disposing of it) and re-inserting the new one. If you have a manual transmission you'll get to do the clutch, too. Price out a used engine here: http://www.greenleafparts.org/inventory/retail.htm This company has at least one operation (B&D) that's a Volvo dismantler; you'll spend less than $2K before s&h.

There are a lot of peripherals to contend with, unless the used engine comes with alternator, etc., still attached. A/c hookups, electrical, vacuum and intercooler lines....these are the details that will eat up the time.

There's a shop near here that puts used engines in FWD Volvos: he repaces the entire subframe with engine, tranny and axles bolted in place. They arrive strapped to a skid. He replaces parts as necessary, does the T-belt, water pump, etc., while it's easy, then lifts it into the car from below. (This business recovers totalled Volvos for owners or direct end users: for many in our area with modest means, it's their first taste of a Volvo at way-below-book prices.)
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(98 S70 T5SE misc mods - mostly lighting - and red calipers) (92 940GLE)