According to API Std.607, 4th edition (1993), pages 7 and 8, paragraph "4.4 Valve Qualification" (see Tables 3 and 4):
- a 4" valve qualifies valve diameters from 3" to 6";
- a 8" valve qualifies all above valve diameters;
- a class 150 valve qualifies classes 150 and 300;
- a class 600 valve qualifies classes 600 and 900;
- a carbon steel body qualifies all body materials listed in Group 1 of ASME B16.34, Table 1;
- a stainless steel body qualifies all body materials listed in Groups 2 and 3 of ASME B16.34, Table 1.
Qualification of fasteners' materials (para. 4.4.8) and "soft" parts (PTFE and other polymers - para. 4.4.9) should also be considered...
I'll add other information and possibly a comparison as soon as I can consult API Std.6FA too.
In the meantime, try also reading Thread809-70969.
Hope this helps, 'NGL