It has been over 30 years sense I ported some cylinder heads. What fun (note: I’ve not missed it, one bit).
Most cylinder heads have about the same casting thickness everywhere. Knowing how thick that casting is, and staying under that, is the big part.
Going too far can be = to having a new expensive boat anchor.
Porting close to the cooling jacket is an area to keep in mind – for those mistakes are deadly to cylinder heads. Coolant is always under pressure and will always look for a leak.
Going too far to the oil area can be fixed with epoxy, sometimes.
Some porters say to get an intake (and exhaust) gasket and port to the size of the gasket. You will find porting that much material off will take forever.
Even this picture of yours shows how much material needs to be removed – if a person was porting to match the gasket.
My opinion (which you take however you want), if doing yourself, port to clean up the roughness in all of the intake/exhaust ports, then stop.
If going serious (full time racing, for example), then send heads out to a professional to have done. That way, if they break it, they pay for it.
I have in my garage, a pair of 340/360 “J” cylinder heads that came off of one the very first 360 2 bbls made. These cylinder heads are the good ones - but the intake ports look like some sort of a Halloween maze in a corn field. The roughest intake port castings I have ever seen. I got some sanding cones and spent a weekend and got 2/3rds of one cylinder head done – and then I gave up (just from removing the roughness).
They sit on a shelf by the door as a reminder- so if I go into the garage wanting something to do . . .
BudW