Your original 4 on the floor is the SM420. Not sure how much torque it can take, but they are known for being stout transmissions. The more modern SM465 4 on the floor will likely take more torque and should be readily available in a bone yard.
If you can find an old cast iron V8 bellhousing that will fit your rear motor mount, then you can swap in the V8 rather easily, use the SM420 and only have to make front motor mounts. Note that the rear motor mount is actually a bellhousing mount, also called a clutch housing mount. Your tranny bolts to the back of the bellhousing and hangs freely.
SM420 has PTO port on driver side, and SM465 has PTO port on both sides. Gearing is similar with a granny low gear. If you stick with the SM420, you won't have to change your tranny cover inside the cab, like you will if you go for the SM465.
Just to stir up the fire a bit... what rear end do you have? Is it original? Is it the Eaton HO72 (perfectly round rear cover on the axel)? If so, you may find that the stock 5.13 rear gear is a little slow for your needs. Finding a 4.10 drop in gear is getting scarce as hens teeth now days. Also, the torque of a built SB 350 X 7.06 first gear X 5.13 rear end = one heck of a lot of torque. The tranny and axle will handle it. You'll spin your tires far before you run out of torque.
