Very true but tire technology back then was very primitive as compared to now. The very best racing tires 1960-70 were not as good as the tires a typical entry level economy sedan gets now. My 2015 F150 truck has “better” tires than my 1970 Boss 302 and 429 Mustangs had. Relatively skinny relatively hard rubber compound tires of the 1960s didn’t “hook up” all that well and 1960s cars didn’t have computer controlled engine, traction, and handling management.
The people that I knew or knew of that drag raced Cobras before they got too valuable didn’t want to get traction too soon. If they hooked up too soon something was going to break including drive shafts, clutches, transmissions, half shafts, and rear hubs. The person I was most impressed with ran Cobras and 427 Cobras. He barely spun a tire as he left the line. He was very quick still but was easy on the original drive trains.

Original street Cobras weren’t engineered for super high torque engines and high traction wide tires. SAI increased the load capacity of all most everything that rotated in their race cars. Ted Halibrand and other suppliers provided higher strength and higher fatigue resistant parts to SAI that just anybody could not go buy at their dealer’s parts department. Even with upgraded parts T-10s were not as tough as some users. If you have read a lot of SAI racing history you will come across tales of some drivers being very rough on transmissions while others didn’t have any problems

did SAI change the racing T10-L from long tail shaft to short tail shaft due to problems with the power train or was it the other way round?

Never heard a question like this before. The tail shaft length (or any external dimensions) for all production Cobras was the same as 1962-64 Ford Galaxie sedan with a 260/289 engine and Borg Warner T-10 transmission. Ford used a variety of “lengths” of transmissions/tail shafts and locations of shifting mechanisms. Only the Galaxie version of geometry was used.

The strength problem suffered in Cobras was that the aluminum tail shaft case was somewhat fragile for some drivers. One can find interviews with Shelby American mechanics dating back to 1964 talking about the problem. It has been published that some drivers didn’t have habitual transmission problems and others had frequent failures.

Some Shelby works team cars, not all, got the tail shaft housings replaced with a cast iron one. From the May 31, 1965 Shelby American parts book, “C2AZ 7039 C Transmission-Housing For Tail Shaft BWT-10-B7 Strong Competition Warner Gear Ea 36.63 35.64”. In later years the Shelby American parts department offered complete replacement transmissions with aluminum main cases and iron tail cases for Cobras.

I like my T10....

I've read about that issue, but can't remember where...