Hi Steve,
As I have found and observed, the proper height of a Big Healey's rear suspension is based more on owner preference and needed ground clearance then official specification. However, keep in mind that my height observations are made within the context and limits of the specific suspension design and components available for application (i.e. spring type, wheels and tire selection, etc.).
Attached. is a driver's side picture of my BJ8 Phase 1 which incorporates the same rear suspension and frame as your BT7. As tiy cab seem. my measurements were made with 165x15 Vredesteins on wires. However, my original springs were reinforced with an extra second leaf to improve stiffness.
It should also be noted that, due to many operational stresses, left and right side measurements may differ slightly and it is not uncommon to see the left side of a left hand drive Healey standing a little lower then the right. Also, since most outer panels are fitted for best match to adjacent panels, it is very possible that one fender may be slightly higher then its opposite when taking measurements relative to the ground.
From what I have experienced, raising the rear height of a Healey is more a matter of component change rather then adjustment. with the axel above the frame and spring attached below the axel, the common way of adding spacers between axel and spring will lower, rather then lift the body. As a result, the only way to raise body height is to either increase spring tension, raise its front mounting point, or increase tire/wheel diameter. Of these, the easiest is mount new tires with spring replacement coming in second. However, all approaches are limited in the amount of additional height they can apply.
My suggestion, enjoy your Healey now. You will find plenty you will want or need to do soon enough.
Hope this helps,
Ray(64BJ8P1)