Hi Bill,
When I sprayed my firewall during blackout I tried to emulate a freehand method the line painter may have done as well as performing it in a fast paced environment. I put a thick harbor freight moving blanket on the top of the cowl to substitute as the "cardboard" John Z. stated was used (and when it was used) to mask it, plugged the hole for the wiper (Mark stated this was done) and then making a few practice dry passes to get the "feel". After adjusting for a tight fan on the gun I then made two freehand passes for blackout and along the top stopping short. I don't recall how far up it went originally when I first restored my car back in the mid-80's but from the many pictures of original pictures I have seen as well as my unrestored LOS, I decided to leave the last few inches only with overspray.
For what you have now you can back tape the existing parts of the firewall and just redo the top part. Unless you can get the DCC mixed the same (this is what I used too, BTW for 30% gloss) so as to avoid a noticeable gloss change, then I recommend to back-tape along the sealer line.
Below are two pictures showing how the top was left.
Happy soon-to-be New Years!
Mike