This method seems unnecessarily complex. If you have libpcap networking, attach xu (4.3bsd de0) to your ethernet interface and you can bring it up (ifconfig de0) and talk to any machine on the local network except the one hosting the SIMH instance. If you set up SIMH tap networking you can talk to the hosting machine (
https://github.com/simh/simh/blob/master/0readme_ethernet.txt). If you set up your router (route add default x.x.x.x 1) you can talk to any host on the internet numerically. Now you can get files via ftp, rcp, etc.
You could also attach a tar file as any block device - RQ, RL, whatever - and have tar read directly from the block device, which does not require any sort of padding or modification of the original file.
I believe that 4.3's tar will understand most tar files, but if you have problems you can always create (or recreate) a tar file using the "o" option to output as an old style archive.
-Henry