Professor Fate said

Did he cut a hole, or is there just no tunnel top? Those tunnels have a removeable top with different lids to suit the transmission that was installed. Pic would be nice.
Manual transmissions used bellhousing mounts and a crossmember under the bell (riveted to the frame) that will need to be drilled/cut out, and a new rear crossmember sourced or built to fit the automatic trans.
Do you have a frame for it? If not, consider sourcing a '72 and up frame- normally these need to be modified in order to get the box to line up, but since you have a stepside (Utiline in Mopar-speak) that's a non-issue. This is a fairly common swap in truck circles. Those with Sweptline boxes usually graft the late model front to the later model rear and splice the frame under the cab, and then all the body lines line up. They are that close! All it takes is relocating the rear cab mounts on the frame and tweaking the front ones a bit.
This way, instead of a solid front axle with two leaf springs, no "true" power steering, huge drum brakes, etc.; you get independent coil spring front suspension, common power steering, disc brakes, much more common engine and transmission mounting provisions, and a frame-mounted fuel tank (the original Sweptlines had the tank inside the cab behind the seat).

Since you're looking at a bunch of fab work no matter which way you go, I seriously would consider throwing a late model 5.7 drivetrain in it, if not a complete late-model frame and drivetrain swap from a rollover or rotted out late model Ram. A bit more work than the earlier ('72 and up) frame, but still very do-able.

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