You didn't mention if they are tube headers or manifiold headers. Most tube headers are specially designed to fit a particular vehicle, i.e. fit around sparkplugs, starter, steering gear, etc., and are not interchangeable. The only way you can tell is to either contact the header manufacturer or (and this is a big or) remove the old ones an try them out for fit. As I'm sure you know, but I'll mention it for the sake of everyone considering changing headers, an exhaust system is exactly that - a system. Simply adding headers will not necessarily increase performance as the header, muffler(s) and the rest of the system components must be selected and/or designed to work with each other to achieve a 'tuned' system. Even if they are, the peak performance may be seen at an rpm range that doesn't correspond to where you need it most. You also need to consider the pre-programmed shift points of the tranny. Changing them to match your new horsepower/torque curves can be done, but involves changing the vacuum modulator and adjusting line pressures - not any easy or inexpensive task. You may be better off with a system from companies like Banks, Doug Thorley, Walker and others, that have taken all these things into condiseration. It may be better all around to port, polish and match the existing headers with the heads.
Thanks for the reply. I decided over the weekend to fork over the bucks and buy a new set that is specific to this M400 chassis. I'm also replacing the pipes and switching out the old single muffler to a set of high flow duals. If I pick up 2 mpg out of this, (fat chance) it will pay for itself in no time at today's gas prices. I plan to overhaul the old 440 this winter and have some bowl work done to the heads while they're at the machine shop. I'll install a new rv grind cam while I'm at it. A little extra pep will help me big time, since I like to haul one of my antique tractors to the tractor shows behind my rv.
Good decision! Also, remember that the firing order is 1-8-4-3-6-5-7-2, so you'll get better overall performance if you install a balance pipe between the two down-pipes near the headers. That's because you get 2 bangs(8-4) in succession on the right side of the engine and 2(5-7) on the left for each revolution; a trick many people neglect. And don't go any larger than 2-1/4" (or whatever the manufacturer recommends) for the down and exhaust pipes and muffler. A little backpressure is needed to achieve a standing wave which helps scavange the exhaust from the cylinders.