I have a rebuilt 1984 302 engine block and I replaced a 1995 351w with it in a eddie bower version 4x4 truck. It sounds strange but it was very economical for me to do this. One problem I've had is that I used the 351's flex plate and now I have some vibration at 1000 rpm and only there. Can I do something other that change the flex plate to correct this problem? Can I use the 351's harmonic balencer to smooth it out? If I decided on not correcting this problem because the truck is only used now and then what would happen?

 

As I recall both the 302 and 351 Windsor engines are externally balanced. Logic would demand that the counterweight on the 351 flexplate is either too light or too heavy for the 302. I'd suggest biting the bullet and put in a 302 flexplate for the year engine, but, before all that, make sure the torque converter is properly centered on the flexplate.