Putting it on top rather than under--the top was exposed and could have been dirty/corroded while the under was clean and gave a better connection. Suggest removing and cleaning it all good (with a fine wire brush) and re-installing.
Also check the ground connections of the new battery cable, It bolted to engine and/or frame. Pull and wire brush them good, re-install. Small wire brush is good, you can find them at Lowes and Ho Depo in the welding section in stainless and I think painting section in brass and steel.
If it still runs rough replace all wires and plugs, with plugs correctly gapped. This includes coil wire. While you are messing with it, consider new coil, points, condensor, and distributor cap if you don't remember the last time you did those. (Also inspect the 12VDC wires (to coil and from coil to distributor) and connections.) All that stuff is relatively cheap compared to the cost and inconvenience of getting towed in from somewhere.
