A few issues here. All FMJ’s got either the large diameter brake booster (like what you have) or no brake booster at all (which might be an option). Finding parts to remove the brake booster is getting more difficult now days – but for most of us, I would stick with power brakes.
Now with that said there are two different versions of brake master cylinders and large diameter boosters. Those with 4-bolt cast iron master cylinders and those with 2-bolt aluminum master cylinders. I’m not sure when the change was made (‘78 or 79) but the 2-bolt aluminum master cylinders are significantly lighter and smaller and is recommended if you have to replace both parts (on an older F or M body).
4-bolt iron master cylinder
2-bolt aluminum master cylinder
The bolt pattern to master brake booster is also different between the two above cylinders.
Some B-body and C-bodies have a smaller diameter tandem booster that will fit (in both 2 and 4-bolt versions – but they are also about 3-4” (75-100 mm) longer).
There are two different K-car brake boosters, single and tandem. Both are smaller diameter than FMJ’s use, with the tandem version a bit longer. Both will fit fine and both versions use a 2-bolt aluminum master cylinder. Front wheel drive cars use a different diameter master cylinder piston – so I would recommend staying with an FMJ 2-bolt master cylinder.
The K-car boosters use metric studs, so a person will need to use metric nuts (4 for firewall and 2 for master cylinder). Many (but not all) reman brake boosters come with new nuts – so that might not be an issue. I “think” the turbocharged cars got the tandem booster – but not exactly sure. Either booster works fine for most FMJ cars.
I have a reman A1-Cordone Brake booster 54-73172 (tandem) in the garage (bottom-most picture), to install into my first big block car – which has more of a clearance problem than small blocks do. RockAuto.com shows price to be $52.00 (US) plus shipping with the single version (p/n 54-74100) to be less ($49) which is the middle picture. If looking either up, you might have to not use the dash (or a space) in the part number.
Factory booster (pictures are not to scale)
K-car single booster
K-car tandem (dual) booster
Note: these apply to most of the ‘80’s front wheel drive cars except for L-body (Omni/Horizon).
The suggestion to modify your valve cover has merit – but before deciding on which method to do, just keep in mind the rear valve cover bolt is almost impossible to access with the large diameter booster in place and for that reason alone, I recommend replacing the booster instead of modifying your valve cover.
BudW