Be aware that a tape head amplifier doesn't have a linear frequency response, neither for recording nor playback. So the original feedback network is not usable for a line input. Unfortunately, I'm in a hurry right now and from a quick look at page 17 of the service manual it appears that the normal/metal switch of the TA7784 is 'abused' to switch from tape equalisation to normal (linear responding) feedback network for the line input.
If so, then the gain in the line operating mode would be ~180 with linear frequency response, set by resistors R408 and R409. On the other hand, the line input is attenuated by ~450 by the divider network R401/402. That would result in a attenuation of ~2.5. No OpAmp needed for that...
I would try the following:
1) Remove the input attenuators R401/402 (100k/220).
2) Remove resistors R410, R411.
3) Remove the TA7784P.
4) Build the attached circuit twice and connect it to the line inputs and the TA7784 pins as shown in the diagram.
Just two transistors will do a better job than any Dual-OpAmp. As an European, I would certainly use low noise BC550C transistors, but actually any general purpose small signal silicon npn transistor will do just fine (such as the 2N2222 shown in the diagram).
Capacitor C1 should be a non-polarized foil type, such as mylar or even polypropylene, but as this is not an audiophile amplifier anyway, an electrolytic will do as well, the positive side has to be connected to the base of the transistor.
Both circuits are powered by pin 1 and pin 8 of the previous TA7784. One circuit has its input from line left and output to pin 2, the other one input from line right and output to pin 16. Line common is now connected to circuit common instead the bias point of the TA7784, hence pins 8 and 9 have to be bridged.
Just a general remark regarding audio grade OpAmps. I have recommended the NE5532 as I initially thought you wanted a playback amplifier, where an excellent opamp is essential for good performance. But now we only need a line buffer, and in this particular case no OpAmp is required at all.