In general I prefer the headers over the stock exhaust. I drive my 45 a LOT, and I wish I had one on mine. F135 headers were never really big. One day I will have a newer engine, and then I will DEFINITELY run a header with it.
Ryan is right about the heat. It is not the header per se, but you do need to spend more time THINKING about how you route the pipes. I got a secondhand exhaust system from MACE that runs REALLY close to the floorboards for clearance. Last year on the way to Rubithon, with ambient air temps at 100degrees, my floorboards measured a smoldering 140!
Lastly, if you still have a column shift, then you've got to do some minor mods to get the linkage to clear the headers. I've done a couple, but its been a while, so I don't remember exactly what I did. Seems to me I straightened one arm, shortened one rod and bent one other about half an inch and it all worked out.
Best
Mark A.
P.S. If you decided to buy a header, I may finally get to sell you something.;p