I'm not sure the angle of the actual driveshaft is as critical as making sure the angle of the pinion on the differential and the angle of the engine are equal.
I found this page -
https://4xshaft.com/blogs/general-te...veshaft-angles
The 2nd video gives a great explanation.
You are going to want to check all this once the whole vehicle is assembled and all the weight is on the suspension.
I'd make sure the back of your transmission is angled down slightly and then adjust the differential with shims once the weight is on. It looks like you are using a factory transmission crossmember that doesn't have much adjustability, other than shimming the transmission up.