Well, the Delta Force never even got to Teheran, and the Spetsnaz might well have similar problems. If anything, the Soviets at that time were even less combat experienced than the American special forces. Of course, that doesn't deny the possibility that they might try it.
Even without help within Iran, the Spetsnaz would have an easier time if for no other reason than they would be mounting this operation from much closer in. Eagle Claw was a two-day opeartion based on planes flying off of the Nimitz, securing a staging base in hostile territory, then flying on to Tehran and rescuing the hostages. The Soviets, with Tudeh assistance, could move across the border and set up a forward base in Iranian Azerbaijan at their leisure. From there, they could mount an attack on Tehran that would be able to count on Tudeh assistance on the ground, cutting off electric and telephone lines and holding off Revolutionary Guards while the Spetsnaz deal with the hostage-takers.
I might agree that Delta Force might have done a better job of freeing their country's hostages had they made it to Tehran, but since the American force couldn't even make it passed Desert One, that becomes a moot point. Since the Russians won't have to face the prospect of a Desert One type situation, that bodes well for their eventual success.
That's why the scenario I propose is based on the Soviets announcing a LIMITED naval blockade ONLY of Iranian ports and ships, and that they will not interfere with ANYONE else. The point is to punish the Iranians, not to strangle the West, and they would have to make that clear. Under such conditions, the West might not oppose them, although I am sure U.S. and NATO naval forces in the region would be drastically increased to make sure that the Soviets don't "step over the line."
Iran's main source of revenue and export is oil, and that was exported out onto foreign ships destined (mostly) for western Europe and Japan. The Persian Gulf is perhaps the biggest shipping area in the world, and Iranian ships make up a miniscule portion of that traffic. Unless the Soviets want WW3, they aren't going to try that option, especially since they could never hope to put enough naval forces in the area to counter NATO.