Kr4...He needs to go mono because of the LF noise of vertical groove modulation, which has no signal, except the noise.
A jumper from TAPE OUT LEFT to TAPE OUT RIGHT might woek as you suggested, and would be the most easy. If not, use two Y-interconnects, 2 to 1 plugged into a 1 to 2, connected from TAPE OUT to TAPE IN, and select the TAPE input. You will need to get male and female connectors or coupler, as required.
Better yet, put a short across the HI wires from the stereo phono pickup. You could tack solder this across the phono input jacks of the amp, or even put a switch in to give you stereo/mono selection for the phono input.
A jumper from TAPE OUT LEFT to TAPE OUT RIGHT might woek as you suggested, and would be the most easy. If not, use two Y-interconnects, 2 to 1 plugged into a 1 to 2, connected from TAPE OUT to TAPE IN, and select the TAPE input. You will need to get male and female connectors or coupler, as required.
Better yet, put a short across the HI wires from the stereo phono pickup. You could tack solder this across the phono input jacks of the amp, or even put a switch in to give you stereo/mono selection for the phono input.