Just use a T-5 with an adapter plate on your bellhousing and keep your present linkage. The T-5 has a 1" longer input so the plate lets you run it with your shallower bell. Many of the Classic Mustang vendors sell the T-5 plates
Remember that the 3.03 three speed came out first and was the initial design used as a pattern for the top loader four speed. And everbody who needed a tough three speed bought it, including General Motors. It was used with 400 Pontiac Firebirds and other fairly high powered cars. And they weren't planning on many warranty failures! So I have to slightly disagree that the 3.03 is not as tough as the 4-speed. And I'm sure that the four speed gear set in the GT-40's was made out of slightly more exotic steel materials than the regular passenger car ones. Of course, also by that reasoning, the three speed gear sets were no doubt made out of less strong material than the regular passenger car four speeds, but the three speed is pretty close to the four speed's strength. When production was transferred to Tremec, that's when the strength dropped. Just my opinion.
If you look at the gear train of the three and four speeds you will stop saying things like the three speed was the pattern for the Toploader. You can't compare the two - the only thing they have in common is a piece of tin on the top of the tranny.
