Modifying RD250C heads for RD350A

[Dvsrd]

Hi all! Great to be back in 2SW 
I found a pair of RD250C heads on ebay, that I plan to modify for my RD350A.
(I already posted on the UK ACRD forum  http://www.aircooledrdclub.com/smf/index.php?topic=58165.0 )

I just assume I could machine the combustion chamber to a somewhat thoroid shape when increasing the volume, since they use 3/4" reach plugs vs 1/2" for 350 heads. Any comments on this?

Then the issue of head gasket arrangement. I have 2 gasket options already on the shelf, O- rings and RD400 copper gaskets. As far as I can see, there is basically 3 options:
One is to machine off the gasket recess on the top of the cylinders, use the 400 gaskets, and use locating dowels to center the heads.
Second is to machine O-ring grooves in the top of the cylinders, and spigots on the heads, for centering the heads in the original gasket recess the cylinders.
Third is to machine spigots AND O-ring grooves in the heads, leaving the cylinders untouched. If the heads are strong enough, this seems to be the most attractive and easiest solution.
Any comments, recommendations or warnings?

Remember, I am talking about modding 250C heads for a 350A. And, yes I am aware that they are bigger, so will need cutting down somewhat. Also, if anyone have pics of cutaway 250,350, 400 heads, that would be most helpful!

[2 Stroke Jamoke]

Awesome idea! I vote for rd400 gaskets. Would be super cool if you pulled off toroidal chambers!

[Jspooner]

I would go with your third option since you will already be machining on the heads anyway. My heads are done that way and it works great.

[Dvsrd]

I would go with your third option since you will already be machining on the heads anyway. My heads are done that way and it works great.

That sounds very promising!
Is that stock 350 heads, or something else? 
I just gambled on buying those 250C heads, assuming they have more "meat" in the dome than 350 heads.
(Possibly the same casting as 400 heads?)

[Jspooner]

DG heads. Scott Clough did mine.

[Dvsrd]

Ok, I guess I need to check out the 250C heads vs unmodified 350A heads, and see how much material I have to play with. I guess the DG heads have plenty of material in the dome area? They are made from an extrusion, right?

[Jspooner]

Yes, an extrusion

[Dvsrd]

Whenever the 250C heads arrive, I'll try to measure the dome thickness vs. 350 heads, and post my findings, and further progress here. It would be great if I could get away with modifying only the heads.
Believe it or not, my near stock 350A is still on 64.00 mm bore, although on the second set of OEM piston after one got holed sometime in the 80s.
And that brings up another question. Is the dome angle on OEM Dykes ring type piston the same as Mitaka, ProX piston? For cutting the squish band.