The body must be forged, but not casting because of the weight of metal and potential defect in casting process.
Please note that I am to present more detailed specification of the valve such as service fluid, material, function.  Could you forgive me because I am looking for the entrance, but not goal.
It looks like you are talking about a very special valve size (NPS 50). You might want to verify if you can not use a 48" valve, this size is much more common.
I however do not understand your comment regarding the forged body. To my knowledge cast steel valve has the same weight as forged steel. Forged materials can have defects, cast material can have defects. The trick is to make sure that you have as few as possible and that they do not have an impact on the valve integrity. As long as you do a check on the material you should be able to use both production methods.

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