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desperate help needed for 1971 Avanti II

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Here we go. I bought this car (RQ1634) in december 21. What with one thing and another, it finally got to my mechanic today.

Turns out it needs a lot more than i was led to believe.

Rear brakes, bushings, power steering pump, a/c compressor, wiring, etc. The biggest thing is the motor. the 305 in it is trash, and most likely the auto trans as well.

I had grandiose plans to fit an LS engine ad a 6L80 transmission, but according tho my mechanic, the motor won't fit. Not without a complete custom set of headers, mounts, etc. and the transmission tunnel is tight, so we need a smaller transmission.

So what have you all done? Has anyone done an LS, or later fuel injected chevy motor, and what difficulties did you encounter?

Am i best off just to find a small block 350 chevy and put an aftermarket Holly fuel injection unit on it?

And suggestions for a transmission?


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LS engines have been fitted but I don’t what mods had to be done.  Hopefully someone here who has done that will fill you in.

When I had a ‘70 Avanti II I bought a crate engine…350/330 hp version.  For my style of driving (I’m not 25 anymore)…I think it was overkill.  The 290hp crate engine would have done just fine plus it’s a regular fuel engine where the 330hp crate engine was premium fuel.  With a 700R4 transmission it could spin the tires in any gear.  

There was minor issue with the 700R4…the transmission support had to be relocated somewhat and a new neutral safety switch/backup switch assembly had to be fabricated.  No big deal but was necessary.  You could go with converting it to a manual 5- or 6-speed transmission.  I’m kind of sorry I didn’t do that but it would have cost more to do it the I would want.

A 200R4 engine would actually have been a better choice…a better spread of gears and could have been built to 700R4 strength…plus being smaller.

I also went with an aluminum driveshaft…less rotating mass and un sprung weight.  I also used lightweight composite leaf springs for less weight.

I went with an Edelbrock EFI which was trouble-free and performed superbly.  Sticking with a carburetor would have certainly cost much less but I wanted EFI.

If I had to do it again I would gone with electric power steering to get rid of all the hydraulics and hoses hanging down under the front but aftermarket electric power steering was just hitting the market at the time and I didn’t care to be a test case using it.

Overall…the best I can tell is to follow the KISS principle…Keep It Simple, Stupid.  Trying to do too much too fast can be counterproductive and expensive.  I would suggest starting with the brake system and suspension first.  Make sure the hog troughs are solid as replacing those eats up budget very fast.  Then the drive train and cooling system.  Interior and body last.

Make a plan and stick to it.  You can get into the weeds faster than you think with mission creep with any car but more so with an Avanti.

Rebuilding an Avanti is an act of love…not of reason.  



Edited by Gunslinger
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On 4/3/2024 at 12:47 PM, ronmanfredi said:

350 with a 200R4.  If you get a high horsepower 350 then you should get a built 200R4 for it.

What he said :)

To elaborate:

350 crate motor - pick your horsepower / price.

2004R because you want a 4 speed automatic instead of the oem 3 speed. 2004R instead of 700R4 because it is narrower and has better gear ratios. 

Edited by Murder
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LS motors have been installed. I'm going to attempt one soonish.

You will need motor mounts to place the engine in the right location . I believe people have used hooker headers (rear dump) to clear the steering box and you need a rear hump sump which Holley and others do.

I have heard that the gearbox fit can be an issue for some but not others so I'm not sure what to do there yet but I'm going to try a 4L60e.

If you have Facebook (Avanti swap page) there is a fellow selling his Ls swapped Avanti. Its rough looking but he might have some answers.


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