Is it safe to use? Probably, and sound will come out if you use the adapter. I'm not generally a fan of using adapters like that with high Gm tubes, but YMMV.
I think what you have to consider is that we have experience with the 5842 and it would have cost less to put the 5842 into the Mainline, but we didn't do that.
The 6C45 has not quite twice the transconductance and a bit more gain than a 5842. This makes the 5842 not particularly well suited for the output transformer in the Mainline, and the lower gain isn't particularly helpful either.
For 5842 use, you can check out the Soul Sister preamp design in Valve Volume 4 1998 for reference.