Join Date: Sep 1999
Location: Vancouver, WA, near Portland, OR
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Re: Short wheelbase needs full sized help!
That would be small internally. Externally all AMC V-8's, (290, 304, 343, 360, 390 & 401), are the same size.
The 304 would be less thirsty than a 360, but less powerful as well. I would not bother swapping a 258 for a 304 as the 258 is a very torquey motor and the gain would hardly be worth the effort, but a 343 or larger is another story.
A built 258 can be a very respectable motor for a SWJ. If the YJ were mine, I'd rebuild the 258, possibly with a stroked crank for more displacement if I could keep the comp ratio below 9:1, use an early vintage stock grind cam, Offenhauser Dual Port intake w/Quadrajet carb. Then run 4.27 or 4.56 gears in the axles for 33" tires. You'd be surprised how much power you'd have and avoid all the re-engineering that's needed for the Waggie powerplant/drivetrain conversion.
Now with that said, I notice you live just north of me, so I'd first find out what you will be required to do for vehicle emissions. Not sure of what's the rules up north, but in So. WA, it's just a tailpipe check with specs set of the original engine's emissions. The advantage of a 30 year old or more vehicle is no more smog checks, that's why I love really old beaters/Jeeps.
'66 Jeepster Commando w/Buick 252/4.1L V-6
'70 Wagoneer w/Buick 350 V-8
'76 Wagoneer w/AMC 401 V-8 (sold)