Oh FFS...
SR71 has already been brought down in the simulator. It is also confirmed that the first model of SA75 obtained a damaging miss on an SR71 in the first year of employment and before missile upgrades were performed.
Stealth can be fired on and hit if it can be detected at a high enough range - IRL this would probably be done with optical tracking, or using data from an IADS if available. The likelihood of a successful interception of either of these 'difficult' targets is fairly low - and improvement in results would come with multiple shooters and a predictable target/flightpath repeated over several days.
Doesn't matter how many B52 there are - you will only have 6 rounds to fire and are looking for 2-3 kills - this is really the restraint the NVA was operating under. Streams of bombers pulverising irreplaceable depots, docks, infrastructure, and in numbers that can only be resisted and deflected by political will, not stopped - "the bombers will always get through".
