I wouldn't have the cajones to ride the 74 above the San Juan exit... Way too twisty and batsh!t crazy traffic.
@Sidewalk - If you drop out at San Juan, and take the road back towards the 74, there is a double gate on the right just before you get to the 74. It will be locked. Jump it, and you're in Caspers. Jumping the gate is probably illegal... The fire road on the other side is perfectly legal. It climbs for a bit past the Hot Springs and then turns into the old Cold Springs Trail (maybe a little bushwhacking) and then the San Juan Creek Trail. This runs right next to the 74 with some ups, downs and maybe a little HAB, but will keep you out of traffic all the way back to the Caspers Campgrounds.
I've seen on Google Earth that the trail keeps going all the way into San Juan Capistrano, but I've never ridden that part. I always head into Caspers and take the West Ridge back to RSM. I have checked out the shoulder on the 74 from Caspers to Antonio Parkway, and there is about 1.5-2 feet of shoulder for a good chunk of the road back to Antonio Parkway. From there to the train station is easy... I just did that on the road bike last week. For you, that would be a quick section...