I was gonna stay out of this one because it's such a can of worms, but I can't help myself...
I put HB on my '68. I'd say quite successfully so. But, it was FAR from straightforward!
Someone told me that HB was only used by the factory when they were painted into a corner. In other words, it's not in there because it's better. It's in there because of space and/or packaging constraints. If you can use vacuum boost, you probably should. But, if you disregard that advice (not necessarily mine)...
I did it because I wanted brakes just like my Bullitt on my '68. There's also the "cool" factor.
I snagged a complete unit off an '03 Cobra. It mounts at quite an angle to the firewall and has the corresponding tilted reservoir. You need to recognize that the pushrod is now on a slope and not level like the factory setup was. If you manage to get it connected to your pedal, you introduce side loads on the piston because the pedal is pushing forward against a piston that is sloping uphill.
You also need to consider pedal ratio. The difference between manual and power pedals is huge. Power pedals pivot up high on the support whereas manual pedals pivot down low in the bushed hole. Since HB applies very different assist than vacuum does, you may need a different pedal ratio.
As I recall, the power ratio is around 6:1. Manual is about 2.5:1 In my 2001 Bullitt with HB, it's 3.5:1 or something like that. So, to feel right, you need to duplicate the ratio of the car from which the HB unit came. But, if you don't have the same brakes, rotor diameter, etc. that all goes out the window.
I am also running Bullitt brakes on my '68, so I duplicated everything I could on the '68.
Getting the HB on the firewall is the least of your problems. I had to remove the factory studs and drill new holes in both the firewall and HB to mount it. But not until after working out where the pedal pin would be and how the pushrod came through, etc.
I cut a section out of my power brake pedal to reduce the pedal ratio from 6.0 (or whatever) to 3.5 (or whatever). Because of the sloping pushrod, I had to move the pivot point rearward in the car so that the arc the pin traveled in was aligned withe the pushrod. To keep the pedal height and position the same as a stock '68, I had to move the hole in the pedal support rearward and downward. This in conjunction with the modified pedal put the pedal in the exact same place in space as stock, but had the Bullitt pedal ratio, and proper movement of the pushrod pin.
This was all done while the car was on a rotisserie and the cowl was out. I can't imagine attempting this (and I wouldn't) in a otherwise working car.
I test drove this system for the first time a coupla weeks ago. Not in the street, mind you, but around in my very large driveway. So far, I have to say, "Mission accomplished."