Welcome to TalkBass, Pc! There are a lot of fans of the Rumble 210 cabinet in the TalkBass
Fender Rumble Club, if you'd like to discuss it there. It's a great bunch of people! Meanwhile, I'll give you my answers here.

A single Rumble 210 cab is 8 ohms, so it should draw
something less than the full 800 watts from the HB Block-800B amp. If it's
something close to 400 watts, then it should be a great match! If it's
something much
more than 400 watts, like 500 watts, then it could be too much for the Rumble
210. But the Rumble
410 should handle it just fine!
It would help to know the HB Block-800B output at 8 ohms to make an informed decision. 
Thanks, Red! Good to know the "800 watts at 4 ohms" is RMS. RMS, and "continuous"
are the most useful ratings. It still
doesn't help choose an appropriate 8 ohm cab, though, without the 8 ohm output rating. The Rumble 210
might be good enough, but it
might not! But, in my opinion, the 500 watts continuous rated (just like RMS) Rumble
410
would be good enough.

Yeah, Nebby! I suspect you might be allowing an extra 50 watts above the published 350 watts rating due to Fender's traditional conservative ratings? But, did you know that
the exact same speakers in the Rumble 210 cab are also in the Fender Rumble Stage 800 combo amp, where they can routinely get up to 400 watts continuous in stock configuration? And, the
same silver grille Rumble 210 cab is also produced with a black grille & hardware to match the "look" of the Rumble Stage 800 combo, where it could also routinely get
another 400 watts continuous as an extension to the Stage combo, in order to reach the total nameplate 800 watts!!

So, yeah, the "350 watt" Rumble 210
can easily handle 400 watts.
Sorry, PcYou, that this still isn't a definitive answer to your question, since we don't know the Harley Benton's output at 8 ohms. But I do hope you find some of this information helpful.
.