That is an answer, but only a partial one. Spicer is generally referred to with the '27' type rear (Spicer or Dana/Spicer 27), Dana 30 rears are always just Dana. Here on, I'll refer to them as Dana 27 only.

What year is your car? The change from the Dana 27 to the Dana 30 happened somewhere around 1966. Mid-'66 perhaps.

Very early cars may have had something completely different.

And wagons are another story. Pre-mid-'66 wagons had a Salisbury rear, which is real close internally to the Dana 27, but not close enough to swap bits.

So, what year and model is your car? And of course, it may have been changed along the way.. so, a quick search here brought up the following from planetman (Eric at High Performance Automotive...reputable to say the least)

"The way to tell the difference is ALL Dana 27 rear ends for Volvos have axle tube outer diameters of 2 1/4" and ALL Dana 30 rear ends for Volvos have axle tube outer diameters of 2 1/2"."

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-Matt I ♥ my ♂