Are the 1WG and 3TJ cases the same, and/or are they all numbered '1WG'?

Can I use a 4JH top end on 1WG cases?

I'm looking to build a 600 engine in 400 cases to keep it legal for Thunderbikes. I have a 1WG engine and a 4JH engine which is why I'd like to throw the two together.

Don't you have to keep the head from the same bike though ???
If you do then the 600 has different volumes to the 400.

All the 400's are 1WG

I'd go for earlier cases / 3he crank / 4JH / top end / over sized pistons