You would be in for a long day to start in San Diego, visit the library and then continue on to Santa Barbara.
You might also consider stopping at Nixon's Library, it is off the 57 and 91 about 6-8 miles from I-5.
As an alternative to SB, you could consider staying in Ventura (30 minutes from SB) at the PierPont. Close to the ocean (have to walk under freeway), and then go into SB the next day.
I do not know the area very well near the Reagan Library, sorry.
Having lived in Huntington Beach I have been to the Nixon library; in fact I've worked all over Yorba Linda and Orange County.
Thanks again!
Actually, it's only a little over an hour away, or about 70 miles.