Not really - missiles don't manoeuvre. A SAM capable of reliably intercepting a B-70 or SR-71 is by default a very capable ABM, but an ABM can't expect to kill an aircraft except by luck or through sheer size of the (nuclear) warhead. Still, it's a useful guide to the size of missile we're talking about - THAAD has the kinetics, but you'd need a different kill vehicle with an explosive warhead and endoatmospheric manoeuvring capability.The types of weapons capable of reliable interception would be similar to those used for shooting down intermediate and lower range ballistic missiles.
Careful with costs - the A-12 family was never truly productionised, so every airframe was a hand-built prototype with significant differences. There would be significant scope for cost reduction. By comparison, the B-70 was productionised from the start since it was intended to be the main long-range bomber of SAC.Depending on how much a strike SR-71 costs, it might be more efficient to go with B-70 type aircraft. The SR-71 cost $8 million each ($96 million was appropriated for 12 early aircraft). The F-12 would have cost $15 to $18 million. A B-70 seems to have cost around $22 million ($265 million was appropriated for 12 prototypes in 1960).
Another option would be reviving the F-108 program to produce the strike aircraft. The F-108 used honeycombed steel instead of titanium, so it should be more suitable for mass production and maintenance by regular Air Force personnel.
In practice, I suspect that the F-108 would always be cheaper than the A-12 due to the design decisions made (steel vs. titanium, J93 vs J58) but would give up some performance to get there.