Yes I thought I was slow and deliberate! Were you driving it at all during that time? I suspect you enjoyed it enough that you were in no hurry to sell. 302/C6? I had a '70 Mustang stick-6 convertible I bought from a co-worker, but when a collector made me a good offer, bye-bye!
The Dan Gurney XR7 were supposed to be good performers. I knew there were some larger displacement engines, for 1968 the 385 series Ford big-blocks at 429 cu. in. were added to be available besides the FE 390/427 & 428's. It was first in the Thunderbird, not the Cougar, in 1968. Later in the 1970's, it was punched-out to 460 cu. in. But small-block 289/302's were probably the most popular Cougar powerplants. Cougars were supposed to be the 'European-style' coupes for the USA, more sound deadening, more luxurious features like premium carpet, special badging, interior and exterior trim and lighting to ensure your next-door neighbor saw it wasn't 'just a Mustang.'
In an interesting offering, Cougar got an electric sunroof, a rare option at the time, and a first for the Cougar. The conversion was done by Heinz Prechter's American Sunroof Corporation (ASC) who did many special option packages like sunroofs and later, t-tops for Detroit. He started out working at George Barris's shop. Barris recommended him to the brass at Ford in Dearborn MI. He left CA and moved to MI. Cars finished but needing the sunroof were shipped to ASC where the work was done, and then sent to whatever dealers wanted them. Prechter made a lot of money with his premium packages, and became someone whose company made such prestigious cars as the McLaren Fox-body Mustangs and Capri 5.0 V8's.
There were a couple of Cougar Dan Gurney cars, available through your local dealer. The Dan Gurney Special, and the XR7-G. Ford was running Cougars in the Trans-Am, prepared by Bud Moore, and Parnelli Jones was one of the drivers. It was 1967.
Tasca Ford in Rhode Island came up with a clever mix of Ford components to make the 390 punch above its weight/cu. in., and that combo was sent to Dearborn MI where Ford engineers tested it, and because it was better than anything they had developed, they made it a factory engine. Tasca Ford won a lot of 1/4-mile races with that, and it made their reputation.
An early Tasca Ford competitor, when the 406 cu. in Ford was offered, early 1960's.
Later Tasca Ford ran altered wheelbase Mustangs, and the SOHC Ford 427 was popular in NHRA and AHRA, after NASCAR booted it from their racing series. Holman and Moody made a handful of extended-wheelbase Mustang dragsters, with the engines moved back so-far, the firewall had to be reworked, and there was sufficient room to stand inside the sheetmetal, in front of the radiator and spun-aluminum gas tank. Hubert Platt was another altered-wheelbase Mustang SOHC Ford 427 campaigner favored by Ford brass.
Here's a Cougar from IN, campaigned by Gerry Schwartz w/a Ford 427 SOHC in 1967. It later ran a BBC and Schwartz died in a racing accident.