Well I don't know why you'd want to use a low power 5.0l block. They only put out 150 hp. I'm sure the 5.0l HO (Mustang, 225 hp) is the better engine block and that uses MAF and not speed density which is what the low power 5.0l have. That plays in to tunability if you are using fuel injection.
But as to what you can use any 1988-1991 Mercury Grand Marquis/Ford Crown Victoria*/Lincoln Town Car to my knowledge they are SEFI. have the same engine and transmission. Good news for you because there a dime a dozen.
Now I would think that you'd be better off getting a engine from a Mustang. Why, I know that not all Mustang engine parts will fit the low power 5.0l blocks. And secondly I believe that the Rustang has hardened pistons and rings. Unless the place that your using can make the low power block more powerful, stick with the Mustang block.
*This car can be equipped with a 351 5.8l V8, 180 hp for all of those years mentioned and has a 2bbl carburator.
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Richard
1995 Lincoln Town Car Signature Series
Past
1992 Lincoln Town Car Cartier Designer Series
1992 Lincoln Town Car Executive Series
1990 Lincoln Town Car