If all that is needed is to concatenate three files into a single tape file, with each new file aligned on a block boundary, isn't (for f in f1 f2 f3; do dd <$f conv=sync; done) >/dev/rmt0 sufficient? The only time I had to do anything like this, the target system was a MicroVAX; hence the tape had to have ISO labels, and the program to be booted a particular tape filename. I don't remember how I handled that; probably I used one of the several public-domain tape-label-writing programs. But that was a VAX; I would assume an 11/73 doesn't have so much enforced complexity in its firmware.