The 1990-1999 Miata seats are a great upgrade for the TR6 seats. You will need to fabricate adapter brackets to use the stock mounting holes in the TR6 floor. Below is a PDF document from the TR Register that explains how to make the brackets and install the seats.

Note: The PDF says that the front right seat bracket should be equal size as the left seat front bracket. This is incorrect, the right (passenger) front seat bracket is wider just like the rear bracket on the right seat.

Miata Seat Conversion\N-WeatherEquipment&Trim.pdf

Below is a picture of my installation.

Below are pictures of John Drosdick's 76 TR6 with Miata seats installed.

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Simon Blackburn's Brackets / Conversion

Simon is making some wonderful brackets to accomplish the Miata seat conversion using the existing holes in the floors. Below are the current brackets he is fabricating. Simon can be found selling these on Ebay from time to time. His Ebay screen name is simonabh.

 

Below are Simon's installation pictures in his car. These brackets are his first attempt and the ones above are his "new and improved". These pictures give a good idea of how the brackets are mounted to the seats.