i only know about the small block stuff- if you have a big block, disregard this.
header fitment depends on what mounts the header prototype car had on it. a 302 and a 350 had mounts that put the engine in one spot- to the right and further back towards the firewall- where a 307 and 327 had the engine in another spot- forward and to the left.
if you have 327 mounts, and the header company used a 302 or 350 powered car to design the headers (which is very possible), then you might have clearance issues around the steering box if you have a 327 or 307 powered car. if they used a 327 car , the headers tend to be more universal.
my Nova is a factory 307 car, and still has the 307 mount setup. cheap dynomax headers clear everything great with no less than 1/4" of clearance at the tightest point at the stearing box. but the Hedmans i put on when the Dynomax headers got bent needed a 1/4" ding to clear.
so, in conclusion, either ding the header and live with it, or buy a set of shiny Jet Hot coated Dynomax headers from Jeg's for $220 or so- or uncoated for $115 or so.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?