To get the cam lifters out, the tappets, pushrods, rocker gear and inlet manifold have to come off as well.
gaskets for these usually included in camshaft kit

But then, literally, 10 bolts later, per side, and the heads can be removed, (well, yes, exhaust manifolds), easy to freshen up the heads, reseat valves (lap) general check.

I would do a compression test before starting, it's simple, just to get an idea how things are, cam wear can give less power, but it happens so gradually, hard to tell if that's the cause of it running out of steam.

I've done a few Rover V8's, with those sorts of miles, it always starts with a simple cam or head gasket change or the like, and ends with full engine out rebuild.
But that's me, can't put things back together knowing other things are worn. Luckily, these old V8's are relatively cheap to rebuild.

It's like opening Pandora's box, how deep does the rabbit hole go?