The size of the bellhousing required for a torque converter might be an issue that requires transmission tunnel mods that impacts footwell and pedal space. There are a couple of 5-speed options that have a history of success. I have one in my '67 +4 that makes freeway speeds much more comfortable. I don't think the tunnel had to be modified at all. One of those conversions might save you some expensive lessons about unintended consequences.
