After looking at this for a while I decided I was far enough along that I should stay the course and get the booster mocked in all the way. It's really the only way I can know how much space I have and need. I think I'm going to be about an inch shy of where I want to be but maybe I can find that and it'd sure be nice to do this on the simple approach I'd been planning.
Need a spacer to located the booster vs. pedals.

Turned the ID of some heavy wall stock to make the bore.


Even using the wood to try to prevent hole saw walk, I still had it move on me. That sucks.

Got to practice my gap welding, but I got it done.


Had to shorten the clevis and thread the booster rod a bit deeper but it does bolt up now.

Flush to the upper skin of the hood. Gotta be a least a 1/4" down from here but at least it gives me a feel for things.

Throttle is fine, but I was about right on calling out the +1" vertical I wanted for the brake. Boooooo.

Well, with the booster pushed full high, I can still shift the pedal up (actually this lets me put it 1/2" forward as well which would be good), but I'm still worried about brake system gain.
If I'm staying the course here, I figured I should at least run some math and scope out brake system gain. Giving up 5-10% on my pedal ratio might not be the end of the world.

The good news is that I think I can get a net system gain similar to what I'm used to in my rx7.
The stock D60 brakes use a 1.25" Master Cylinder. If I drop this down to 1.125" to up the hydraulic ratio that puts me in the ballpark, but I have to hope the pedal feel doesn't suck. I'm hopeful as I found a gent who talks about doing something damn similar:
http://home.4x4wire.com/erik/4runner/brakes/ Looks like it's a relatively proven setup all the way up to 44" tires:
https://www.pirate4x4.com/forum/toyota-truck-4runner/61436-d60-under-toy.htmlNote, this guy was using a Toyota booster but it also looks like a dual diaphragm that's the same general size and class (8+9"). Two differences between his math and mine. 1) He compares hydraulic ratio vs. a single brake caliper vs looking at ratios vs. both sides. I added the row to my spreadsheet called 1/2 Gain to compare apples to apples. 2) He's just looking at hydraulic ratios where-as I also calculating gain of the entire system from brake pedal to COF of the pads, to leverage of the tires. The method is from the book "High Performance Brake Systems - James Walker" if anyone is curious.
The references to available boosters from the 70s and 80s GM 1/2, 3/4, and 1ton trucks were helpful. Notably, the old school master cylinders with integrated reservoirs are shorter in most cases than the plastic versions and I need that to keep the booster high.
The last issue is the MC to booster pin interface. For the life of me, I can't find a means to search MC's by attributes and half the time they don't even give me bolt spacing much less details on the booster MC interface. I'm debating buying the more expensive Wilwood booster (
https://www.summitracing.com/parts/wil-260-8556/) just so I get the MC "bullet" insert that I can easily machine for a variety of depths. Oh and for the record GM used 3.220 and 3.400" bolt spacing on the old school MCs. The Wilwood comes with slotted mounting to work in a range from 3.220 to 3.400". If I go cheap and buy a 76 Blazer MC I'm going to have to machine it open a great deal further since I'm measuring about 3.150" on the Subaru booster and the Blazer is one of the 3.400" versions.
-Joel