The exhaust manifolds usually crack, but what's wrong with the intake manifold?
Exhaust manifolds are kinda hard to find because they usually crack. Headers are the easy option. Intake manifolds are not as hard to find (I have a couple).
The Downey header will bolt right on. You may need to use an angle grinder to open up the slots for the bolts a little bit.
I have a Downey header on my 55 and I had one on my 40 for several years with no issues- worked great. Make sure you buy the CERAMIC coated header. The first header I had was not ceramic coated (chrome) and cracked after about a year. I also pulled a cracked, rusted, not-ceramic-coated header off the 55 after I bought it.
The exhaust may need to be reworked where it attaches to the flange on the bottom of your header. A muffler shop can do this for you easily.
The header should also give you a little more HP and MPG.
It bolts right on as well as any other header...be prepared to "fit" your header to your intage manifold. It will take some work, they all take some work to fit. You'll want an angle grinder, and a dremel, and maybe a couple of the good gaskets and some extra washers.