Well good luck with the build, I really worry for you about the incline required, you cannot have less a difference than 80mm so you get a clearance of 70mm allowing for thickness or boards and track ballast/underlay.
As Robert says anything less than 1 in 30 is a killer so you must avoid that, I use 1 in 33 which is OK, now in order to have room for the hand of god (you) then anything less than 132mm less 12mm board giving 120mm clear is a bother, so to get up this gradient you will need 4356mm plus transition curves at say about 200mm as a minimum, in other words to get up this height you need 4476mm which is 14 feet and about 8 inches, - you do not have this length so the design is made to climb 80mm only even then it would need 2840 mm or about 9 foot 4 inches. In the plan they talk about 'absolute minimum heights of 75 mm which I think gives no strength, my steamers are up to 67mm so this leaves nothing and gradients of 1 in 68, hmm looks a bit unlikely to me.
In that case you will not get into the fiddle yard, so sorry to say I view this plan as a bit of a Cyril - looks OK on paper but no one has built it,
My advice is build this end first, one part solution is to lift the left side by half the height and this will reduce some gradients but make other ones climb where they are flat now. On my 1 in 33 gradients by no means does every loco perform well and the decoder chosen can allow runaways in some cases, so you need to do a lot of testing as you go. The other thing is that you will need access, once laid the upper main lines jig saw cut the boards so they can lift out and you can get underneath, I suppose you could always build a rack railway........