So I bought a 1975 pontiac 400 to put in my 1966 Pontiac Grand Prix that had a 389. Got  a great deal on a rebuilt  400 and  the motor mounts are different, What  frame mount can i use to make it fit correctly? Any Good advise would be appreciated. 

A 389 block and a 400 block are external the same. However, I think your issue is with the engine mount holes (that are drilled and tapped). They do not line up, correct?

 

Blocks made between 1970 and 1976 had five holes so that blocks could be used in both the new and old models. 1977 to 1981 only had the three holes. So you may have to drill and tap the proper bolt holes if they are not drilled and tapped already.

1959-1969 Blocks have only (2) motor mount holes on each side of the block (see #2 & #5). These blocks will fit any 1964-1974 Pontiac A-body except cars originally equipped with a 455, which used a 3-bolt style mount. Later cars with 3-bolt motor mounts can be adapted to fit this block using a Butler Performance WRP-MM6 or Ames Performance N24CCA adapter kit. The adapters are available but not a perfect fit so expect some swearing / adjusting when installing.