258,000 And still running strong...sort of.

My '88 K1500 Sportside has the 305cid TBI and currently has 258,000 mi. on it and runs really well. There is a puff of smoke at initial startup after sitting overnight, I figured it is probably valve guides. Not a real issue
at this point in time.
What I am debating about is whether to keep the 305 and have it rebuilt when the need arises, or to scrap the 305cid and go 350cid or 383cid. The 305 with 258,000mi is all stock and has never been cracked open for major work. So a bore and rebuild would be fairly cost effective, but on the other hand.... a bit more get up and go would be nice.
Does anyone have any opinions on the advantages vs. disadvantages would be...... for example... cost, mpg, complications during swap etc....
I currently get about 15 to 17 mpg. What could I expect with a 350cid or 383cid?
I definately want to keep the truck for a while, it has never been wrecked, never bumped....(with the exception of 3 door dings about the size of a dime) and has absolutely no rust in it anywhere. Everything in it works with the exception of the oil pressure gauge, which I believe to be a bad sending unit. I am only the second owner with all paperwork.
Any input would be appreciated.
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My 99 Z71 with a 5.7 liter averages 14 to 16 mpg, but I have 3.42:1 gears in it. It depends on your final gearing as to what your mileage might be. A 383 motor would be more of a fun ride I think but probably won't last as long as a 350 or rebuilding your 305. But the 5.7 has plenty of get up and go. I would imagine that you need a computer for the 5.7 or the ability to reprogram your with a swap to a larger motor. I don't know if the TBI parts are the same for that year with different motors you would want to check that out.
Brian
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RE/

How many miles does it have on it?
--
PeteCresswell

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It's got 66,000 miles on it.
Brian
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