IMO a four speed transmission will significantly improve the performance and enjoyment in driving your y-block powered vehicle.
IF you see yourself doing any amount of highway driving, like Canadian Hot Rodder, Mark (lyonroad), and Al (slick56), I also would recommend that you consider the Tremec T-5. The 5thgear reduction in rpms will prolong the life of your engine and save you gas money.
Modern Driveline has some excellent information on their website at www.moderndriveline.com
In looking at their Internet listing, they have six topic headings. Toggle ‘Tech Alley’.
Within the listing that opens, scroll down to the seventh topic, ‘T-5 History’ and toggle it.
The T-5 has been in production for over 30 years. The short story is that ALL T-5s are NOT the same. Some T-5s you DO NOT want. The design has evolved and been improved overtime. Well into the Modern Driveline article, they have a section devoted to the T-5z and explain how it has the best of what’s been developed for aftermarket performance use.
Also, on the left side of the ‘Tech Alley’ section page that opened with all the topics, is a category, ‘Specs’. If you toggle this heading it will open to a page of data regarding all of the Ford application T-5s that have been produced over the years with serial numbers, vehicle applications, torque ratings, and gear ratios.
The T-5z Mark mentioned has a 2.95 first gear and comes with a .63 overdrive from the factory. It’s rated to handle 330 ft. lbs. of engine torque and up to 450 horsepower. Unlike many other three and four speed transmissions, the T-5 weighs about 75 pounds! The BW T-85 overdrive I’m using in another vehicle weighs 150!
IF you were to deal with Modern Driveline for a transmission purchase, they offer alternate overdrive ratios of .73 and .59 that they will install for a fee. Based on my vehicle’s intended use and the rear end ratio I’m using, I went with their .73 overdrive ratio offering.
P.S. The ’93 Mustang Cobra and the ’94 and ’95 Mustangs have a longer T-5 input shaft that has to be swapped out to work with the y-block bellhousing.
Hope this helps.