79 ford f150 4x4 power brake issues

Anyone know what would be the next option to try ?  My 79 has a 400m that was rebuilt 40k ago by the previous owner who knew nothing about engines and had it done for him, so i only know what i can see on the outside. It has a weiand 4bbl. intake and a 600 cfm edelbrock intake, long tube headers , it idles smooth so i don't think it has a big cam in it . My issue is the brakes on the truck, i have went over every inch of the lines, replaced calipers, wheel cylinders, had proportioning valve rebuilt, replaced the booster and mast. cyl. but still the brakes give feedback through the pedal and feels like i don't have enough booster pressure. i checked it with a guage that showed 17-18 inches of vacuum. My question is that i've heard that if i modify the brake line fittings at the master cylinder that i can bolt on a 79 f350 2wd. booster/master cyl. that has dual sections to store more vacuum. Or i could buy a comp. cams electric booster set up and use it to supplement what i have . I love my truck but if the brakes are never any better i might have to get a newer truck. Don't want a payment though. Thanks for any help/suggestions you guys have.