Estes SR-71 Blackbird Build Part 14, 24mm Conversion?

If you've ever seen the SR-71 Blackbird fly you know stability is a concern. I've seen them fly perfectly vertical or turn and go horizontal into the wind. Installing a heavier engine in the rear could add problems.

Before gluing in the engine mount I thought about converting the SR-71 Blackbird for D engines.
The model is certainly heavy enough, the face card say 3.2 oz.
I compared the weight of the 18mm mount with a C6-5 installed.
The total weight was 1.04 oz.

A new D12-5 engine and a 20/50 engine block weigh in at 1.69 oz, more than a half ounce more.
The recommended engines are a B4-2, B6-2, B6-4 and C6-5.
A B6-4 and C6-5 have a maximum lift weight of 4.0 oz.

Because of the stability questions, I'll play it safe and stay with the 18mm C engine!