In the upper right hand corner of the forum page is a box "Google Custom Search." Type in a specific word or phrase to pull up many of the past topics which you may find covers your questions. Here is a post on the 455 engine mount problem: https://www.gtoforum.com/f178/69-gto...engine-106922/
As you raise the engine higher, make sure you are not putting the fan blades into the fan shroud or binding your exhaust pipes up against the floor. It may also affect clutch linkages unless you have swapped for hydraulic.
I can't help on the pinion angle other than printed materials. But, if the trans angle is -3 (downward) degrees, the pinion angle should be in the range of +2.5 (upward) or so to match, but not be enough to create a "0" driveline angle. Here is a good explanation and simple diagram added:
"What is important are the front and rear operating angles. The operating angles are defined as:
Front operating angle
: the angle between the output shaft of the transmission and the driveshaft.
Rear operating angle
: the angle between the pinion gear and the driveshaft.
The front and rear operating angles must be equal
and as small as possible without being zero.
A little more angle in the rear may also be needed to compensate for axle wrap under acceleration. The amount will vary based on suspension components and the type of bushings. So some experimentation may be needed."
And from the internet is another explanation and How-To: Pinion Angle - Wolfe Race Craft
To adjust the pinion angle, you would need the adjustable upper control arms. However, you might just try the car as is and see if you get any vibrations out of the driveline and then go from there.
I suggest you invest in the upper/lower control arm braces. This will help stiffen the mounting points. If you still have the 10-bolt, it may not hold up if you drive the car hard as you say. But then again, beat on it until it pops, then worry about it.
Keep us posted.