Can I include variables in an HTTP::respond string?

would it retain the spacing between the variables in the string?

yes

e.g.

[root@ve11a:Active:In Sync] config  tmsh list ltm virtual bar
ltm virtual bar {
    destination 172.28.20.15:80
    ip-protocol tcp
    mask 255.255.255.255
    profiles {
        http { }
        tcp { }
    }
    rules {
        myrule
    }
    source 0.0.0.0/0
    vs-index 34
}
[root@ve11a:Active:In Sync] config  tmsh list ltm rule myrule
ltm rule myrule {
    when HTTP_REQUEST {
  set var1 "12345"
  set var2 "abcdef"
  set var3 "xyz"
  HTTP::respond 200 content "$var1 $var2            $var3\r\n"
}
}
[root@ve11a:Active:In Sync] config  curl -i http://172.28.20.15
HTTP/1.0 200 OK
Server: BigIP
Connection: Keep-Alive
Content-Length: 29

12345 abcdef            xyz