Hate to say this but, on a special order, you could get the close ratio or ultra close ratio (i think that's what it was called) on any V8 mustang....I have the ultra close ratio in mine, with the original order docs and...its been in the family since it was ordered in 1965.From the factory The only 64-66 Mustang to get a close ratio would have been a 289 Hipo "K" car. That said I had a 66 coupe for a while that had a T-10 in it with a tag that said it was coded for a 66 Mustang (was it factory,I don't know I bought the car in the 1990's). For what it's worth I never had any issues with the trans ,it was a seemingly solid good smooth operating transmission.
Hate to say this but, on a special order, you could get the close ratio or ultra close ratio (i think that's what it was called) on any V8 mustang....I have the ultra close ratio in mine, with the original order docs and...its been in the family since it was ordered in 1965.
the only close ratio only cars were the big block cars that got the big input shaft toploader. the factory ford big input toploader never came in wide ratio. Dan Williams toploaders and David Key toploaders make a 1.375" big input in wide ratio. back in the mid 1970's 2 different A code mustangs i owned had the close ratio toploader. i found that out when i put Schaifer clutches in and put the trans in 1st gear and turned the output shaft 1 turn and the input shaft turned 2-1/3 turns. i bought wide ratio toploaders and put them back in. toploaders were easy to find at the local wrecking yard for about $100.00. my other A code mustangs had the wide ratio. tremec made the gears and they were turning out an equal number of both close and wide. i have come across about as many close ratio toploaders for sale as i have wide ratio trans for sale. a friend has a 2 close ratio toploader and 1 wide ratio both from mustangs and both small input shafts. right now i have 2 close ratio toploaders and 2 wide ratio toploaders both from mustangs. if you look at David Keys toploader i.d. chart you will see the close and wide ratio toploaders and the cars they came in. since ford used the 5 code for all three 4 speeds in 65-66 there will never b any way of knowing what came in the car. i could take a wide ratio out of a 65 mustang biult in march of 1965 and put a close ratio toploader in and as long as the toploader tag has HEH-BT on it , it will be correct for the car. if you want to be accurate on the date code get one that matches the build date on the door tag.
the 2 mustangs that had the close ratio transmissions were bought from the original owners , one being an elderly woman. the wide ratio is usually prefered by drag racers and for the street. i use a close in my roadrace 65 mustang and a wide in my 68 torino cruiser. back in the 70's i lived in jackson mississippi and there were a lot of hotrodders there and many many 65-66 mustangs. the best guy to get toploader info from is Dan Williams 828-524-9084. Dan started his toploader rebuilding service back in the 1970's and the family also owned a ford dealership.I was hoping Beechkid would chime back in as I'm very intrigued by this and was hoping to see some verification on this,for my own education. As far as Close ratio only early Cars 64-66 Hipo Mustangs (Not Shelbies) with stick shift all left the factory with Close ratio toploaders. As far as close and wide being equally common I would beg to differ,I come across in my travels through swap meets,ebay,craigslist, junkyards about 100 or more wide ratios for every one close ratio that I find,to that end if anyone has a HEH-BX (66 close ratio )Trans I would gladly trade two or three Mustang numbered wide ratio Mustang tagged transmissions.I know what you're saying about the cars you bought back in the 70's having them,but wouldn't you say it is possible that they were swapped in prior to you buying the cars. I mean I have a friend who's dad was running around in the early 70's in a 65 G.T. Coupe that he swapped a(I cringe) chevy 350 into.