Picture of 340/360 head porting....Mistakes?

On my bench, an ootb X head is better than an ootb J head 90+% of the time.

They start out better, and with the same effort applied to both, they end up better....... at least that’s how it works out when I do them.
It’s strictly because of the shape of the floor, and what you can do without fear of hitting water.

The X head only came on 340’s..... which means it only came on a HP factory engine...... and it only came with 2.02 intake valves.
So, the shape of the floor was designed for that sized valve...... and the bowl to accommodate it.

The J head was designed to be able to accommodate either a 2.02 valve...... or a 1.88.
There aren’t 2 different castings for the two different valve sizes....... it’s one casting machined two different ways.
Which means they had to add enough material to the floor in the area of the short turn to make it usable with the smaller valve.
The approach from the floor into the bowl on a J with a 1.88 valve is pretty similar to the X with the 2.02.
When you add the 2.02 to the J, a lot of material gets removed from the bowl at the short turn, which makes the transition from the floor into the bowl more “stood up” and abrupt.

When you compare an X to a 2.02 J, the floor of the X appears noticeably “layed back”, and you’d have to do a fair bit of grinding to the floor on a stock 2.02 J to get the same form as the ootb X.