Wilwood makes a 7/8" bore master cylinder. These run about $300 for the lowest priced one, but should bolt up to your adapter plate. I am unsure of the outlet sizes, but they are inverted flare.
Also, the late 80s, early 90s, front wheel drive dodge/plymouth/chrysler compact cars (Shadow/Sundance/LeBaron) came with a master cylinder that is pretty close in looks to the Strange MC. These cars came with master cylinders in 3 different bore sizes . From smallest to largest is the 21mm, 7/8", and 24mm bore. The outlet sizes for both front brakes (rear port) and rear brakes (front port) is 3/8-24. If you are using your stock brake lines, you will need adapters.
These master cylinders are now manufactured NEW in aluminum and cast iron.
The advertised size and the actual size might be different. For example, an advertised 21mm bore MAY actually be a 7/8" bore. An advertised 7/8" bore MAY actually be a a 24mm. The 24mm is usually the only one that will be aluminum and the advertised size will be the actual size. The 21mm is rare, but these usually come in cast iron, but not always. The 7/8" bore can be aluminum or cast iron, but it may be advertised as aluminum, but it still may come in cast iron. The 24mm master cylinder is usually aluminum.
The caliper casting for the g-body is the same as the third gen f-body. 1978-1981 calipers where NON low drag and used a strait bore master cylinder. 1982-1988 LOW drag and used a step bore master cylinder. A step bore master cylinder can be used for any year NON low drag and LOW drag caliper, but a strait bore master cylinder cannot be used with a LOW drag caliper as it cannot supply the volume needed for the brakes to function correctly. Good news is that any remanufactured caliper SHOULD BE NON low drag as the part numbers are all the same for all the years, but I am NOT sure about your 1987 calipers on the front.
A 1970 to 1980 H-body (Vega, Monza, etc.) came with a 7/8" bore master cylinder, but the mounting bolt spacing may not work with the adapter plate you have. These master cylinders are cast iron and have the GM outlets. Front brakes (rear port) is 1/2-20 and the rear brakes (front port) has 9/16-18.
Hope this helps.
Let me know if you have any additional questions.
Dave