SFX is not an infrared film. Not at all. It is important to get that idea out of your head from the start. It doesn't capture anything beyond the spectrum that you or I can see with our own eyes. The difference is that it is simply sensitive to deeper reds than most films, which do not capture the deep reds that our eyes can see. In essence, it is like expanding your palette...but you are still only using visible light. Read the data sheet, and you should be able to at least arrive at a starting point for filtering this film for the desired effect. If I remember correctly, the human eye/brain can sense radiation up to about 750 nM, most pan films are exposed by radiation up to around 650 nM, and SFX is exposed by radiation up to around 740 nM. Basically, you get an extra 100 nM of red sensitivity, which is why they call it an extended-red-sensitivity film, or a near infrared film. If you only want to make use of the extended range, you will get a filter that only passes radiation above about 650 nM, such as Heliopan RG665, if my memory serves me well as to the number. If you only want to make use of the very top of the extended range, a Hoya R72 will give you about a 20 nM range to play with. If you want to make use of the extended range, plus some of the warm end of the normal panchromatic range, you will use regular-old warm filters such as orange, red, deep red, etc.
