It's not a difficult job, but the manifolds are heavy. It's best if you
work with another friend, two is even better, and install the risers
after the manifolds are on the engine, less weight and in some cases it
makes it easier to get at the bolts. If the engine and manifolds are
fresh water cooled you may not need to replace the manifolds, they last
a long time if fresh water cooled, just replaceing the risers should be
good enought. Check the hoses to the risers while your at it, they may
be worn, this might be the best time to do those as well, fresh hoses
are usually more flexable and much easier to get on the risers than the
old ones. Replace the hose clamps on the risers as well, it's best to
use the "T" clamp style on the exhaust hoses, they last forever (not
quite, but close) but make sure you get the right size.

John