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The ZZ-4 engine, GM #24502609, is a direct bolt-in for your application. You will need to change the waterpump to a short leg style. Also, if you wish to maintain your stock, mechanical tachometer, you'll need to swap out the HEI unit supplied with the ZZ-4 and replace it with your stock distributor (hopefully, rebuilt and as-new). You absolutely MUST, however, transfer the distributor drive gear from the HEI to your old distributor. Or, you can just install a GM #10456413 distributor drive gear on your original distributor.
If you wish to make the installation have an overall configuration more like your original, you can, optionally, do the following:
1) The ZZ-4 engine uses center bolt style valve covers; your original are perimeter bolt type. You can use adapter set GM #24502540 to "convert" the center bolt heads to perimeter bolt style. However, the valve covers will appear slightly "higher" than stock because of the thickness of the adapters and the fact that the rocker cover rails on these heads are somewhat higher than the original heads. Still, the "conversion" will look a lot more original than the center bolt head valve covers;
2) replace the composite timing cover used on the ZZ-4 engine with a stamped steel version as originally used on your engine. It MAY be necessary to do this, anyway, with the short leg waterpump. The stamped steel cover you need is GM #10243967 and the timing pointer is GM #3991436;
3) replace the oil pan with oil pan GM #10055765. The overall configuration of this pan is identical to the pan originally used on your engine EXCEPT for the fact that it's designed for the 1 piece rear seal engine and right side dipstick that the ZZ-4 engine uses. The pan that comes with the engine will fit your car just fine, but its configuration is considerably different than the original pan. The sump will also extend lower than the 10055765 or your original pan;
4) paint the engine Chevrolet orange after any or all of the above changes are made. The ZZ-4 is supplied with bare aluminum heads, bare aluminum intake manifold, and black painted remainder of engine.
I have done the ZZ crate engine swap in my original owner 70 with turbo 400. You will need to switch the supplied flex plate on the ZZ4 engine to a larger flex plate. GM part #12554824 is the heavy duty flex plate that will bolt up to your turbo 400. This change will also allow you to continue to use your current starter. Another GM crate engine to consider is the 350/330 engine, it comes with the vortec cast iron heads.
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