I don’t have specific info about the swap. Those model numbers are generic and cover many engines. If you listed the specific engine numbers it would help.
You could get the factory model number of the 8.5 cam and google it to see what drawings that cam shows up in…i.e….if it ever shows up in a 358.
An LH318SA, HMSK90 uses 909303B for an external cam.
An LH358SA, HMSK110 uses 911300
An LH358SA, HMSK110 uses 911361B
So even the LH358SA’s can use a different external cam.
If you have both available you could mic them.
I’ve swapped out a a few 5hp’s and an 8hp but they were all similar models and years.
There’s a Tecumseh Yahoo forum and you would ask there. T-man used to hang out there and was a super source for info.
tecumseh_engine_group@yahoogroups.com