I didn’t check in any this weekend and have a lot on forums to catch up on.
BruceNZ, this thread, post number 42 - I hadn’t read that article before, but have read it in a lot of older magazine articles before. A good read/write up.
Other than chamber size, the ports are close in any pre-magnum 340-360 - but the “X” heads are best untouched. With minor porting, any (non-magnum) 360 head will flow about the same, if not better, than a 340 X head. Magnums (and aftermarket) are the exception.
An unmentioned item is: to use a ~’75 or newer 360 head, one will need to tap and plug the air injection holes, if one is not using an air injection exhaust manifold (ie: need to plug holes if using an older exhaust manifolds, magnum exhaust manifolds or headers).
This job is not hard to do and is reversible (if needed).
On Torque Converter stall speeds. This is a trick question – for knowing what stall speed you want, you also need to know car weight, engine power and
quite a few other details - for if the exact torque converter is installed into a super lightweight car will have a different stall speed if same (exact) torque converter is installed into a large bus.
Also, the stall speed on an existing converter
will go up as the engine power
goes up.
There is a torque converter measurement (memory is thinking K.U., but could be wrong) to compare converter “A” to converter “B”. A lot goes into figuring out what converter one actually needs. Also changes to vehicle can/will change the size converter one actually needs.
The point I'm trying to say is i'm not exactly sure if your replacement will be what you want. Then again, it might be exactly what you want.
If making engine changes - I would hold off on converter replacement until you can retest the stall speeds (if able to wait, that is).
The low gear set and/or lockup between the 904 and 999. The A904/A998/A999 transmission case is the same for all 3 transmission types. The /6 uses a different bolt pattern case and the A500 has a different case (for 4th gear/overdrive).
Now with that said, a person can change any A904/A998/A999 transmission to lockup or to non-lock up.
They can be changed from the older low gear set to the newer gear set (IMO, the better one).
Also any A904/A998/A999 transmission can be converted to the newer style lockup clutch (if wanting to retain lockup, that is), to the more reliable converter hub that has two flat sizes, from the slotted hub.
Just about the only thing you can’t do is convert an A904/A998/A999 case to an A500 (overdrive).
Using a lockup converter (vs. non-lockup) gets the average person about 9-10% better fuel mileage. Most of that is highway driving. City driving, it does help, but not as much.
Personally, I am building a 727 for my big block, and I will be converting it to lock up. On the same note, I am also planning on driving it 30k miles a year – so the expense to convert it, will pay for itself.
There are pro’s and con’s both ways on going to (or retaining) lockup or going to (or retaining) non-lockup.
Personally, I prefer lock-up. That said, don’t use my opinion to make your decision.
To recap, you can use both of your transmissions (the 904 and 999) to make one that has both the gear set you want and non-lockup.
I wish you were closer (like a couple hours’ drive) to me, and for dinner and a few cold ones, I would come down and get the transmission exactly like you want it (except for the Removal/Reinstallation part).
BudW
Oh and I will feed you as much NZ beer, Wine, Water as you can take along with good NZ food! And you can drive the doba to get around if you think you can drive a left hand drive car on the left side of the road, now thats crazy! try not driving on the other side, its quite hazardous to your health! Oh and while I am at work earning money to pay for the expensive parts as I don't like the idea of selling my ball sack to pay for for parts you could bolt the trans in and out 
