It looks like all the 120 sedans came on bias-ply 5.90-15's (early ones on tube type, later tubeless) until about '64 or '65, then they went to tubeless 6.00-15's. At some point a few years later, the sedans went to tubeless 165S15. Notice there's no "R" for radial in that size designator, just a change from 6.00 (inches) to 165 (mm), still bias-ply tires. The '67 123GT got radials (165SR15), but apparently the regular sedans never did... at least not in the US (I had two new 2 dr sedans, both were on bias-ply whitewalls). Meanwhile, it appears the wagon never had anything but 6.40-15's.

In addition to the bias ply stuff, Coker does pedal several 165-15 radials, from a no-name sorta $50 or $60 blackwall, all the way up to a Michelin XZX 165SR15 (almost period correct), for something the other side of $125 IIRC. I agree with the earlier posts... if you're going to stay with a "stock" size tire, definitely go for the radials. The bias ply tires are way clunky by comparison. :)
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Gary L - 1971 142E ITB racer, 73 1800ES, 02 S60 T5
BlueBrick Racing

Funny enough, as I read the original post, I was looking at the 1965 Volvo 122 S brochure that arrived in the mail minutes ago.

According to the brochure:

Rim Size, cars, 4Jx15"
Rim Size, wagons, 4 1/2Jx15"
Tubeless Tires
Size, cars, 6.00-15"
Size, wagon, 6.40-15"

I would concur with the previous poster that modern tires would make a lot of difference (all for the better, I think) in the handling. Although too much rubber on the ground up front (or significant change in wheel offset) could make for harder steering.

Ben