14" Disk brakes
Why ? I got drums all around on my 68 Valiant with 400 hp works fine.
If you're pushing 400hp and have never needed anything other than stock drums, well, that 400hp is being completely wasted. It's not hard to overdrive a set of stock drums with 400 hp, not hard at all.
Yes everyone knows front disc are better, but is he upgrading because the drums aren't adequate there not on par with his driving style, have they failed regularly/once and cause near misses my guess probably not ? Rear disc aren't a needed mod.
If OP just putt around town drums are fine. If he does high spirited driving though the twisty and or long steep down hills with lots of braking then yes, especially if they maintained and not giving the performance needed. But if it's just to do something to the car it probably isn't necessary but it's his money and there's worst things to spend it on.
Ah the old ignorant rear disks aren't needed opinion.
Rear disks brakes have been shown, again and again, to shorten stopping distances. Are they needed? Well, if you need to stop in say 122 feet instead of 133, then yeah, maybe they are? A lot can happen in 11 feet.
Mopar Muscle did a rear disk conversion on a '73 Dart Sport and compared the stopping distances between the rear drums and rear disks from 60-0. Their result was that from 60 mph factory disks up front and factory drums in the back the car took 133 feet, 6 inches to stop. After the rear disk conversion, the stopping distance improved to 122 feet 4 inches. It wasn't a super scientific way to test it, but it was better than most of the documentation out there. And the car involved was set up the same way as a lot of these cars, with larger rear wheels than fronts.
The online article is a bit of a mess now, probably something with being moved from MM, to Hot Rod, to MotorTrend after each of the previous magazines was bought out. The final distance is a caption on the second to last picture if you open up the images for the article. The hardcopy article is easier to follow, but that's the way it goes.
https://www.motortrend.com/articles.../?galleryimageid=274576?galleryimageid=274576
I'm not saying they're mandatory, or even that everyone needs them for how they use their car. But if replacing BBP rear drums with a
basic set of rear disks can knock 11 feet off your stopping distance, well, that's more than enough to make it a good modification. And that's not even considering the SBP 9" or even 10" rear drums, since they both have less stopping power than the BBP 10" rear drums.