Would a 1976 mercury 400 engine fit in a 1966 mustang?
The 400 is an 'M' block, Modified, because it was a take-off on the 351 Cleveland block. The 'C' block had some bottom-end short-comings, so a fix was tried, which was the 'M' block. Didn't work. It's a good solid low rpm utility engine quite popular in the LTD II series, as a police & taxi engine. There are almost no aftermarket parts available for it, because it was only around like two years. You're best bet is a 'W' Windsor block, which includes the 289, 302, 5.0, & the 351W. Tons of parts available, & the 351 W can be stroked to 427 cubic inches.