If you aren't into burn-outs and power shifting you can use a t-5 transmission out of a third gen F-body. The better ones are good for more than 300 ft/pounds of torque. I installed a world class t-5 in my '69 about 5 years ago and have been very happy with it. Best part is that good used ones can be found for less than $500. Requires shortening the driveshaft and moving the console back 2.5". Original 350 crossmember can be modified to work.