how much are the ARB's worth
Around the £100 mark I think, very rare indeed now adays.
unfortunatley I don't have the correct (early) gearbox casing to mount my engine in
You can fit the MK5 gearbox housing to your box (if that makes sence

) as it uses the same mounting as the MK3.
Someone like
www.gearboxman.co.uk will be able to change it all over fairly cheaply.
It was only the MK4 escorts that used the cross member type of support.