You might check that the transmission downshift switch is working right. On 67s, that switch also controls the high/low stall speed of the torque converter. At idle and under heavy throttle, it is supposed to switch to high stall speed. Under mild acceleration to constant cruise conditions, it should be in low stall speed. If is not shifting to high stall speed at idle and is staying at low stall speed, that would make the direct clutch hookup when shifting into drive or reverse harder than the high stall speed will.
Here's how you can check your downshift switch. Your switch has 3 terminals on it, one towards the back and a pair in the front. The terminal in the back is common, and will have 12 volts applied to it; it may only be there when the key is on. With a test light you can check that the switches are making contact; if you don't have one you need one. Of the pair in front, the lower horizontal one is the downshift terminal. It should make contact when you open the throttle roughly halfway. Below about 65 - 70 mph, this forces the transmission to downshift to a lower gear. The forwardmost vertical terminal is the one for the switch pitch converter. At idle, that switch should make contact. Through moderate throttle openings it should be open. Then under heavier throttle it should close again, and that would happen just before the downshift terminal makes contact. When the converter solenoid has 12 volts applied to it, it should go into high stall speed. If no voltage is applied to it, it will stay in low stall speed. Obviously at this point, if the switch isn't making contact, you will be stuck in low stall speed. That might tend to make the car lurch hard when it is shifted into gear. It will also be a little hard on your acceleration; a higher stall speed will increase your acceleration a little bit, as well as pull the car a little less when idling in gear. This is just an idea; the car lurching hard may still be something else.
This applies directly to '67s. The '65s and '66s also had a switch pitch converter, but they had a different carburetor as well, and I don't know if the downshift switch might be a different configuration. However the general operation would be the same. The '68s, like mine, have the same downshift switch as a '67 minus the third terminal for the converter, as '68s and later no longer had a switch pitch converter. And I don't know what style of downshift switch was used after '68. But again, the general operation for a forced downshift would be the same. Let my know what you find. If you determine your switch isn't working right, don't throw it away just yet. I described a fix for these switches in a post a while back. I could post it again if you need it. If you can find a switch in a wrecking yard, I would scarf it up. These switches are absolutely unobtainium.