The biggest issue will be getting the torque right on the head bolt assembly.
Would suggest being extra carefull on cleaning out and lubricating the threads in the block. Torque by the book with a least some kind of heater bringing the block and head up above freezing. Then be sure to retorque the head warm after first running itup to operating temp, also by the book.

John
V-performance.com

I've assembled several engines in mid-winter, unheated garage, around 0 fahrenheit, no problems. Not that much fun, but if that is when you have the time... I don't think the 20 degrees makes much difference to the hardware, just to us. The heavy duty mechanics on the oil rigs and in the oil sands work outside all winter, which might be -40 around here. They do heat their tools!
--
67 144, 86 740T, BMW R69S, R60/2

The longer you wait, the more likely you are to lose parts. I'd say go for it. Of course, thats me... I've done so much damn car work in sub freezing (often sub zero F) temps, I figure someone else should have a similar experience.

I rebuilt my Jaguar's engine and brakes in a small unheated garage over the three coldest months of the year shortly after I bought the car. Miserable work, but I had it ready for spring...

--
-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC

I agree with George, cold is hard on the knuckles! -30C will not effect the strength of the steel as much as the flexibility of the torque wrench. Make sure everything is coated in light oil to keep out the rust. Wait for Spring or get a heater to get the garage up to 10C above, for your comfort. Keep the mice and squirrels out of the parts, especially the wiring.

Klaus
--
98 V70R awd(101Kmi), 95 854T(85K mi), 75 164 (176K mi), 88 245(165K mi)

Thinking more of you than the car but you should try to get some heat going
in there. I don't recommend setting the car on fire while you work on it
but there are portable heaters that are relatively safe.
I think I might be reluctant to torque headbolts in very low temps (well below
zero, for example) if there is a way you can avoid it. Steels exhibit a
ductile-to-brittle transition at below-zero temps and the temperature where
this happens varies widely. During WWII it was responsible for a number of
"Victory ships" breaking up at sea. Since then steels have gotten cleaner
and better but you have to get special steels to have them guaranteed still
tough (ductile) at -75°F, for example. I would expect Volvo to be better
than most in this respect, but if you can warm it up even to freezing you
will be doing yourself a favor all the way around, I think.
Sure glad it's not me!! Although our nice Indian summer with temps
in the 60s ended while I was in church this morning. Went in my
shirtsleeves and was really chilly going OTL 2½ hrs later!
--
George Downs Bartlesville, Heart of the USA!