Re: How do you know if a mach 1 is a true 428
Block: C7AE-6015-C or F or C8AE-6015_R or C8ME, Back of block (bell housing area) should have a weld mark shaped as a "C". Block has extra casting webbing next to the crank mains, thicker main bearing caps.
Heads: C8OE-6090-N (known as Nancy heads)
If your car is a SCJ (not CJ) it should have 4.11-1 (Y code) or 4.3-1 (W code) gears.
SCJ (not CJ) also have the "drag pack" option, external oil cooler, both horns monted on RH side, AC was not an option.
Many other areas to look out for, but these are the main areas to look out for.
1970 Boss 429 Hemi owner