72 custom/10 5 speed conversion

[72Custom/10]

i have a chance to buy a 5 speed transmission out of 80's V8 camaro. i would love the final gear ratio of that transmission to put in my Custom/10. it has a 350 with a 4bbl, intake, cam, and headers. i think the T5 transmission will give me much more speed and gas mileage. i'm wondering if the original 3 speed and the 5 speed are the same size. if anyone has already done this conversion, let me know how u did it and what it took to get it done please.