Contempt for one's enemies is often dangerous...
Urban,
I tend to agree with you that the PLA was no match for the Far East divisions that the Soviets had in place, but ANYTHING that large deserves at least some degree of respect. The Red Army is at the end of a very long strategic tether, and for it to actually 'win' a victory of any significance (remember, the goal here isn't to pile up 'victory points' killing Chinese, as viscerally satisfying as that might be for planners) they are going to have to do more than just run tanks over infantry formations. The Chinese demonstrated a very effective resistance capacity against the Japanese in WWII, and this time around they will have significantly improved preparations for that sort of war.
Keep in mind that Soviet tanks (for Far East divisions in 1969, we are talking about T-62s, for the most part, with early-gen BMPs and BTR-60s) had substantially difficulties that weren't discovered until much later (though the Six-Day war in 1967 should have been a good hint...pity the Soviets pretty much ignored it), and the effectiveness of mass conscripts in this sort of war would be quesitonable at best. For the most part, Chinese resistance from inside of towns and villages (where RPGs and grenades would make effective, if not optimal anti-vehicle weapons, not to mention recoilless rifles, which the PLA had aplenty) would make combat expensive for the Soviets, and they wouldn't have the option of simply bypassing all points of resistance. Without Harbin at the very least, no war with the Chinese is going to be worth the cost, and Harbin would be an extremely difficult proposition, and an expensive one even if they get there and seize it. All of this includes the risk of one or two Chinese nukes 'getting through' and taking out a critical target (Vladivastok and Irkutsk are both possibilities, and neither are trivial in terms of their impact on the battlefield, much less the overall strategic position of the USSR post-war), not to mention the (remote, I concede) chance that a Chinese IRBM hits something valuable in European Russia, which could have all sorts of long-term effects that the Soviets wouldn't find worth it.
Not to say that the Soviets wouldn't try it, not to say that they wouldn't THINK that they could win, not to say that they wouldn't win, even win big time. BUT...and it is a very big but....there are some serious risks, and to simply brush off an army of that size fighting on its home turf with adequate (not great) preparation...holding them in contempt is a mark of very poor planning, and very, very dangeorus indeed.
Finally, the NVA would certainly suffer badly from the results of any sort of Sino-Soviet war, but the US would be very unlikely to do much to take advantage of it. The war wasn't popular at that point, and it is more likely that Nixon would use this as an opportunity to force peace on American terms. The VC were a spent force (Tet finished them as anything other than a mask for NVA activity), and without sufficient supplies, the North might very well have considered making peace with an eye towards rebuilding to fight another day.
Ward,
There is little doubt that the hardware available to the Chinese would have been significantly inferior to what the Soviets would have been fielding (remember, the Far East military was often better equipped than even the Red Army's 'European' forces), but this really doesn't mean all that much. The T-62s and Mig21s (and now Mig-23s and various Sukhois for ground attack) weren't all that good to start with, and while much of the equipment available to the PLA was in fact Korean-war vintage, that particular vintage aged well. The PLA would have been outclassed tactically, but they could trade space (and lives) for time, something that the Soviets simply could not do. A very bloody war, particularly one without a decisive finish, would have been a serious problem for the Soviets, and this doesn't even begin to take into account the possibility of a nuclear strike.
You rather blandly suggest that the Soviets would have chosen to use nukes right off the bat, and you might be correct about this. Certainly they would have used chemicals, they had never made any secret about that, and chems would be ideal to use against a largely infantry force like the PLA. With that said, nukes offer a big risk, as there is no guarantee that the Chinese would not 'get lucky' and hit a high value target inside the USSR such as Vladivastok or Irkutsk, either of which would seriously complicate the war effort. This ignores the (admittedly remote) possibility that something much worse (a Chinese IRBM hitting Moscow, or some other Soviet slum) which could be catastrophic no matter what the outcome of the war. I don't say that the Soviets would categorically reject leading off with nukes, but it would certainly have been a very tough call...