I'm just about to chop another one up in the next few weeks so appreciate your apprehension
Fortunatly, I've got access to a Bandsaw and a couple of hard lads to help but otherwise I would suggest a Jigsaw and plenty of masking tape.
As for dimensions, then it's up to you really. You need to decide how you want the bonnet to look but if you look at Ians, it looks about the same as mine. I have mine covering down to the last point the Bulkhead crosses which makes the support.