Just to be clear, you are changing the 3.8L to a 4.3L? If so, what vehicle are you removing this from? I assume something like an S10?
If this is the case then it is not going to be easy simply because the 4.3L was not offered in this vehicle so it is not going to just bolt in after changing the motor mounts.
You are going to have to use what are called universal mounts or so as you are suggesting, cut them out and hang the engine and line up the mounts from the 4.3L, mark them and weld them in.
Then you are going to have to have the transmission from the other vehicle and mount that as well unless there is a bell housing adapter that will fit your transmission to the 4.3L.
Then you will need to get it running so more than likely you will need the PCM and wiring harness out of the other vehicle as well.
Please let us know what questions you have or if you need other info and we can go from there. Thanks
Tuesday, April 6th, 2021 AT 5:45 PM