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Depends what shape the lower end is in. It could, but for how long I don't know. You see plenty of motors with around that HP running for a couple hundred thousand.
120K miles for one engine isn't the same as 120K miles for another. The real issue is, how well maintained was the engine? Since yours is still running OK, it has probably been maintained pretty well. But, bearings, valve guides/seals, rings, etc. do wear out.
If you are OK with the likelihood that throwing a new upper end on your engine is going to yield problems down the road...and you will do a complete rebuild at that time-- Why not? You can still reuse the top end stuff when that time comes.
The least common denominator will end up breaking. If the top is worn out, that will probably go first. If you make the top strong, rings or something below will go first. Agree w/ 71, top end now followed by bottom later or just do it all now.
I did the top on mine as soon as I bought it and immediately starting loosing a lot of oil though the rings.
I'm in a similar situation with my'74.....she runs real strong but I have NO idea how many miles are on her as the speedo didnt work when I bought her....says 46000 and is working now...
I know it needs top end work as I get the tell-tale puff of valve guide seals smoke on start up. Goes away after a couple seconds and doesn't smoke after that.
I considered putting a top end kit only but don't wanna blow out the bottom end.....
It's possible that you can do a cam/head/intake swap on your motor and be just fine. But you'll need to be prepared to pull the motor if you find too much of a ridge. Add to that, you will probably want to pull the motor anyway and solve the factory-installed leaks and 30 year-old seals.
Honestly, 350s are a dime a dozen - some careful shopping on craigslist should net you a complete, rebuilt motor with the heads you seek for about the same money as heads plus shipping.
Of course, once the motor is out - well, you do have a body lift, right
I would pull the engine anyway to clean and paint...
The main reason most folks do a top-end is because you don't have to remove the engine from the car. If you are going to be pulling the engine anyway, it's worth it to dis-assemble the bottom end, and at the very least, hone the cylinders with a dingle-ball hone (if the cylinders are in good enough shape to get away with not being bored) and put it back together with new con rod and main bearings, and new rings. It'll be easier to clean and paint the block this way, too. It's a little more time and effort, but it's not that expensive, if your block doesn't require any machine work.
The main reason most folks do a top-end is because you don't have to remove the engine from the car. If you are going to be pulling the engine anyway, it's worth it to dis-assemble the bottom end, and at the very least, hone the cylinders with a dingle-ball hone (if the cylinders are in good enough shape to get away with not being bored) and put it back together with new con rod and main bearings, and new rings. It'll be easier to clean and paint the block this way, too. It's a little more time and effort, but it's not that expensive, if your block doesn't require any machine work.