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If you are changing from an inline 6 cylinder the engine side mount brackets will need replaced. The frame mount is the same for both 6 and 8.

334971, 334970 and 459021 are the original factory part numbers. These were used on 73 - 79 A, B and X body cars if looking for used ones or the new number which is still available 10213125 are the only engine side brackets I know of that will work properly when installing a V8 into a 75-79 Nova. The numbers are stamped into the bracket. Right and left are the same. I have compared the different parts and all are the same, the different numbers are probably part number revisions over the years.



The GM part number 10213125 is available from GM parts direct for under $25 each + shipping or can be purchased at any Chevy dealer for around $35 - $40 each. Left and right are the same. You may also want to change the rubber mount that bolts to the frame. These are available from any parts store under various part numbers. The ones in the photo are Parts Plus brand part number EM-2292. I do not remember exactly what I paid for them, around $10 each. I decided to go with polyurethane so they won't be used on this car, but the shells will as the poly mount does not come with the metal shell.



The frame mounts, again right and left are the same and when purchasing stock assembled mounts there are no problems. However the poly mounts are not manufactured with the steel shell. When placing the poly into the shell you have 2 create two identical mounts with the ears facing the correct way or the engine brackets will not drop in.
In this photo are the stock set and the poly inserts. On the stock set notice there are different spacing on the mount holes. Now look at the poly mounts and notice there are different angles and postions of the locating tabs. It is possible to install the poly mounts wrong in the shell. When installed properly in the shell the lower of the two tabs will be on the end of the shell with the wider spaced holes. Assemble both shells to be identical and if using the correct engine side brackets the mounts will line up correctly.
Click here for the how to on converting to the poly mounts



The bolt does not support the engine it only retains it. The tabs on the rubber mount fit into the notches on either side of the bolt hole on the engins side bracket. The 2 pieces need to be a matched set for the bolt to line up. GM made several different configurations of the clamshell type mount. The biggest difference is the height of the engine side bracket to correspond with different frame configurations. The tabs and slots they fit into were made different so only the correct mounts were used in any given application.
 

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Very good information. Thank you for posting this info.

I have a 78 Olds Omega SX and it has the V-6 in it. The frame stands and engine mounts for it are different (as they use a solid frame stand and a rubber/metal engine mount). I also if you have the Olds 350 engine in your car, then the mounts are also different. I have seen the Buick skylark with the v-6 as well.
So if you want to put a Chev v-8 in the Olds or Buick to replace the v-6 or Olds V-8, then just buy the mounts you have shown us.

IF you want to put an Oldsmobile v-8 (like a 350, 403 or 455) then you need Oldsmobile stands and mounts. I know the Cadillac Seville 76?-79 have the Olds 350 fuel injected motors in them (at least the 78 does) and are a K-body which was built on the X-body platform. Many parts are interchangeable. The frame and engine mounts will work for using in our X-body cars since they are on the same sub-frame.
 

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the rubber cushions and engine block brackets from a 77-79 Trans Am with the Olds 403 (which used a Pontiac cushion (frame side) and a block bracket which bolts 2 the 403 will interchange - with the Olds-powered 75-79 X bodies the mounts were also shared with the Seville but the part #s have been discontinued
 
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