French nuclear forces at the time, like those of the U.S., can be viewed as a triad: Land-based missiles, sea-based missiles, airplane-delivered. From this chart, France had anywhere from 36 to 235 nuclear weapons during the 1970s. I’m not sure how accurate that is; when you go through and look at the specifics of what entered service when, it seems really low for the earlier part of the decade.
Anyway, on land in the 1970s, France had two different sets of missile systems, intended for different sets of targets. Battlefield targets were handled by the Pluton missile system. Max range of ~120 km, one warhead of around 15-25 kT yield. So, not usable against Soviet cities, but adequate against invading Soviet armored columns. Total deployed according to the wiki, about 60 missiles max.
The land-based citybusters were SSBS S2 (S3 IRBM in 1980). There were 18 of them, each with one warhead, in hardened silos on the Plateau d’Albion in SE France. Range was ballpark 3000 km, enough to threaten Moscow, Leningrad, all of the Ukraine, Volgagrad, etc… Yield was 120 kT for the S2, which bumped up to ~1MT for the TN60 and 61 warhead that initially capped the S3
The main countervalue deterrent was at sea with the Redoutable-class SSBNs. Four of them were commissioned by 1979, each with 16 SLBMs. The SLBMs were initially the MSBS M-1 and M-2, (armed with the MR-41 500 kT warhead, per Dr. Carey Sublette, and M-20 (TN60/61, 1MT yield, ~3200 km range). Basically, single warhead IRBMs with around a 3000-3500 km range.
Finally, France had freefall bombs dropped either by Mirages or the Étendard naval strike aircraft. I thought their ASMP (sort of a SRAM AGM-69-lite, but ramjet instead of solid-fueled rocket) was in service then, but not according to the wiki.
So, leaving aside whether Mirage IVs could make it into the Soviet Union, the French had 18 land-based nukes to threaten Soviet cities with, and anywhere from 1-4 times 16 submarine-based nukes.
It seems to me that there was an interesting game theory problem with either the U.K. or French submarine nuclear deterrent. Specifically, if either the U.K. or French wished to sit out an incipient Warsaw Pact/U.S. strategic exchange, how could they guarantee that the Soviets would not mistake an, e.g., U.S. Poseidon or Polaris launch as coming from a French or British sub instead? In the case of the U.K., AIUI, the missiles were largely identical to those in U.S. service. The French missiles weren’t, but to Soviet early-warning satellites, wouldn’t they all just be submarine-launched missile launch signatures? I don’t see a way at first glance that the British or French could allay Soviet fears that either W. European country would renege on a promise not to attack the USSR. Well, other than by directly informing the Soviets where their own subs are. Which runs the risk of the Soviets instantly trying to destroy those subs.
Anyway to give MHO about the OP, I think you’d need to establish an existential reason for the Soviets to have attacked: something that had an absolute certainty of having the Politburo and their families swinging on ropes to induce the Soviets to start hostilities that had every chance of turning rapidly into a strategic nuclear exchange. I mean, what would it take to threaten you before you start thinking that flirting with global thermonuclear war wouldn’t be worse?
I think the figures floating around the late 70s-early 80s of five days post-invasion before strategic nuclear release were insanely optimistic. Both sides knew that, both sides felt that any nuclear explosion would lead to 10s-100s more fast, neither side wished to experience it, and so neither side wanted to do the one big thing that would probably kick it off. Besides, from the Soviet Union’s POV, world communism and socialism were on the rise in the West and the Third World, so why screw up that progress with a shooting war?