It's called a 3.flow,
not 3.slow
Is this an auto or 5-speed? How many miles are on the motor?
One of the best things to do for a tired motor, however costly, is to get a fresh valve job done. That will brighten it up a lot and you'll see much improved MPG, easier to start, smoother idle, better bottom end power, minimized run-off during shut down, and have an easier time passing smog (if this applies to you).
It just depends how wore out the engine is. A quick compression test will yield the needed answers to this question.
Also, with a fresh top end you'll have a solid platform to start throwing aftermarket parts at and the motor will realize more gains from said parts too
After I blew the head gasket on my ~200k 3RZ-FE 4-cylinder (long story), I had a valve job done before slapping the head back on and WOW what a difference! It was a night and day difference, seriously, but this was because unbeknownst to me it really needed it. Starts easier, purrs like a kitten, is also more stable at near stall conditions during Rock Crawling too. It made a huge overall improvement for me.
Of course you've got two heads so the cost is higher (I highly recommend Genuine Toyota head gaskets!), but if you are serious about this, then I'd start with a fresh top end (it's short and less expensive than a full rebuild anyhow) and then work my way up!
BigMike
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