I have been daily driving my Ivy green 65 Convertible in Southern California for a couple of years now. I have been researching seating options, looking for seats with improved bolster, back support, and a headrest. Ultimately, I decided to go with a pair of classic Recaro LSC seats. I had them reupholstered to match the stock style pattern and piping with aftermarket seat heating elements. There are a couple of threads on this forum about the “Recaro Idealseat, ” which was made by Recaro beginning in 1973. The Recaro “Idealseat” was available as optional and standard equipment by a number of automobile manufacturers using an assortment of different parts composed of the “L modular system” seat backs, bottoms, and headrests. I absolutely love these seats and think they fit very well within a Gen 1 classic Mustang.
Apparently they were available in the 79-83 Mustang Pace cars and an option in GTs. They were also standard equipment in the BMW “E21” from 75-83. My particular pair of seats have the LS back in conjunction to the adjustable C base. The LS is one of the most popular seats Recaro made through the late 70's/80's/90's. Variations of the LS were found in but not limited to cars made by Ferrari, BMW, Alfa, Volkswagen, Porsche, Ford, and more. They are most commonly found in mk2 volkswagens and Alfa Romeo Milano Verdes within the US. The LS was available with an a,b,c, or d base, but most commonly found with a b or c base. The C base has a retractable base, so it is in my opinion the best version. The Recaro LX is one of the oldest Recaro variations available along with the LS. It is commonly found in early to late 80's cars such as the TransAm, Mustang, 300zx, and various other models.
Installation into my 65 convertible was surprisingly simple requiring a pair of ProCar Seat adapter brackets #81266 bolted to the seat riser floor using 5/16 in.-18 RH 1.5 in length hex bolts and stock style nuts. I used a pair of Recaro seat track sliders - inside bar with tabs #0.70.9. I probably could have found a cheaper generic set on Ebay, but these literally bolted right in. From the factory, Recaro offers a version with outside sliders, but I opted for the inside version and used a 7/16 spacer between the seat base and track slider. The tabs on the sliders are great because they offer some seat height adjustability. For example, I had my seats on the highest tab in the back (to go low) and had the front mounted to the mid tabs (increase height) to give the seat base a type of reclining angle. The Recaro Idealseat has a fully adjustable reclining back with locking latch for safety. Something as we know, stock 65-66 Mustang seats do not have. The headrest has height and angle adjustment as well. I also added three point seat belts and welded bungs into the lower inner rocker and into the interior rear metal quarter panel. The convertible interior is narrower than a coupe or fastback due to the additional inner rocker panel, but I had no fitment issues with the inside slider style tracks. These seats fit perfectly between a stock console. I love these seats! I’m considering picking up a second pair for a 66 Fastback. Also currently having the rear seat reupholstered with matching upholstery and heated elements.
The Recaro Idealist has become wildly popular and prices have steadily risen over the years. Recaro has brought back the “Idealseat” by making their own authentic reproductions as the “Classic LS and LX.”

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TMI makes reproduction seat foam for the LXC style seat.
https://www.cjponyparts.com/tmi-seat-foam-recaro-gt-1979-1982/p/SF19-V/
LMR has an informational seat guide
https://lmr.com/products/mustang-fox-body-seat-guide
Recaro Seat Track sliders 0.70.9
https://www.summitracing.com/parts/rec-0-70-9
ProCar Seat Adapter Brackets 81266
https://www.summitracing.com/parts/sca-81266
Recaro Seat and track slider thread is M6-1.0x 30mm