While also trying on the Y tail fitting for size, I did decided to fit and pilot drill the fwd cross ties on the aft fuselage. I did have to trim/grind the ends of the cross tie box assembly to get the 40" dimension from LH skin breakline to RH skin breakline called out in the plan. I'm within a 1/16' of it being 40" and I guess that will have to do. I don't want to trim anymore, because then I'd be getting close on the edge distance of the cross tie box fasteners.
The plans don't call out a specific place to start, so the best plan I've come up with is to start at the forward cross ties and work my way back. I'll test fit the lower skin to make sure that the lower side is squared correctly.