My thoughts exactly Alan. The (slight) problem with cutting the send signal to neg 10 is that you loose the noise advantage of working at the higher level. There might also be an increase in source Z, certainly will be if the original outputs were configured as "zero impedance" drives.There is a lot of reading here which I admit I have not read all of it, but by not using the tape returns you are making the routing of the 3500 less flexible. Why don't you just make up a balanced to unbalanced +4 to -10 pad for each tape input and connect to the tape returns? The pads are very cheap to make, just a few resistors, if you have a patch bay you could even build it into the back of that. This is how I have my M3700 wired and it works just fine. All the info here.
Alan.
Also, anytime you are forced to send unbalanced signals a long way try to use a "shielded return" system. In this the audio hot and return are included in an overall shielded cable but THAT shield is connected to mains earth, not signal. I have a 20ft run from a Teac A3440 of all 4 tracks and the hum on the line is at -93dBFS, rather better than the hum from the heads! That is using CAT6 FTP cable BTW.
Dave.