Like you, I have hoisted the dinghy out of the water/alongside using the mainsail halyard
or another one, although the dinghy was considerable smaller than your old and new one.
The 25Hp enduro would weigh a little over 50 kg/110 lbs, and that would add up 320+110=430 lbs or about 200 kg.
You didn't say what size/type of boat
you have (catfisher 28????), but I imagine that if a yacht or cat is big enough to store such tender
, the halyard/sheave/mast will be strong enough to support that.
The only caveat is that the halyard
tension would not be in line with the sheave and the sheave could cut into the halyard, and on a cat this misalignment would be even bigger. Regular checks for chafe might be called for.
Edit: oops didn't see the "block on lower spreaders"........ I doubt if many spreaders are designed to take such downward load, OK, not all the 200 kg would be hanging on the spreader, but..... Maybe if the intermediate shroud
is terminated on that spreader with a turnbuckle?