There are two styles of sb bracket. The 1st design is used for the non-interlocking motor mount, 2nd design is used for the interlocking motor mount. 1st design used on all 67-68 sb & 69 307/327, 2nd design used on 69 302/350.
2nd design is narrower & shorter than 1st; unfortunately the non-interlocking mount will fit it. Since that mount is wider and thinner, the engine will be positioned too low and move around, causing vibration and clutch chatter.
My guess is you have non-interlocking mounts on 2nd design brackets. The correct mount is a very tight fit on the bracket.