This is tricky. Both SFO and LAX are busy airports, and flight delays are frequent. I'm flying SFO-LAX route a lot (on Delta connection mostly, though) and from my (bad) experience half of times the flight is late; the worst I've seen was 90 minutes late. The way it usually works, your plane arrives at SFO late due to bad weather in Portland, you lose the takeoff slot and have to wait, and then you circle around LAX until you get the landing slot (credit to LAX ATC, we have never had to circle more than twice). However if you take an early flight from SFO (such as 6am flight), the plane will be there and your chance of success are high.
Then, you have to change terminals in LAX. Bus ride is relatively fast (as @pnuts said), your main issue would be security lines. Unless you have Clear/Star alliance gold or fly in business, you might be stuck for 30+ minutes there (and more if you are pulled in for extra security screening, which itself can take 30+ minutes). Note that TSA Pre does not work on international flights. Add 30 minutes for time to bus, on bus and from bus, and you don't have that much buffer time.
I'm also unsure you'd really be checked in on both tickets. You might not need to transfer your bag yourself, but you might need to stop by the check-in counter, and show them your luggage claim tag to make sure they pick it up. I had this situation with Delta/Aeroflot flight on the same ticket - apparently Aeroflot did not receive this information directly from Delta. Almost missed that flight then.
In short, it is doable if everything goes smooth, but you don't have much buffer time there.