Question about a old mad bypass repair

So several years ago my son and I did the " mad bypass" on his 70 Duster, at the time instead of using fuseable links we used two ATO style fuse holders with 40 amp fuses, sometimes one of the 40 amp fuses blows, you can always tell because the volt gauge only reads 12v when it happens. My question is...is a 40 ATO fuse not enough? Not equal to the fuseable link, OR could we have another issue? BTW once we replace the fuse, it'll be fine for several days usually.