As that thread's first post talks about, knock sensors work on frequency, so you want one that operates at a frequency range you except to operate at.
I installed one in my Buick 455 V8 from an early 90's Chevy truck figuring low rpm loads, etc. I drilled out the existing water drain plug threads in the block and re-tapped it for a larger thread that matched the knock sensor. Same could be done on the 250. The early EFI Buick 3.8's installed them in a hole on the flat spot where the transmission bellhousing bolts to behind the intake. I don't think Chevy's have a flat spot there though.
I never got the one on my Buick to work properly because of valve train noise, but I think the MegaSquirt software has gotten better to where it will only listen to the sensor during certain sections of crank rotation to listen around valve train noise, but I haven't re-tried it though the sensor is still there.
