looking to make my truck motor and trans reliable

[Firebirdjones]

The old 350/350 combo is about as dependable as it gets so long as everything is in good working order. It's the most common engine/trans combo out there, millions made and parts are practically at every street corner.

The only troublesome area would be the HEI. Being electronic there is the possibility of modules that die with no warning. I've always carried a spare in the glove box on any HEI car. Easy road side repair if it happens.
Everything else is mechanical on a 79 and generally won't give you issues. For the trip I'd toss a new water pump and fuel pump on there for piece of mind if that stuff is more than several years old. (about $25 a piece so it's cheap insurance).

LA to Oregon is a fairly easy trip (I-5 all the way) with no mountains to speak of. Doesn't really take that long. I drive Prescott to Sac quite a bit (with mountains). Your truck will probably make the trip just fine.