I used the one mentioned from gnttype.org on my cover and worked for my 3.5 THDP. All I used was a 4" grinder and a 4" flap disc to smooth it down. Came out great.
I intended for people to use a photocopier with zoom, and just play making copies until the 5" line was 5". Or, if you have a drawing program you could scale it there and print it. That drawing came with the thdp I bought from ModernMusclecar back in the day; I didn't have a scanner so I just took a digital photo to send to a friend and it seems to still be going around the net . Glad it turned out so useful.
drew a sharpie line on mine told my local welder cut it ... I think hes cut 3 so far for me an welded on 2 duttneck intercoolers when sprayed black look stock