I don't know what the heck Yellow Rose is squaking about. I do think the main studs are a bit over kill. But what ever makes you sleep soundly at night. Better off over built then under built and over abused.
I also have a '79 Magnum. It currently has a bone stock '00 - 5.9 with bolt on's in it. No cam or head swap yet. RPM intake, Hooker Super Comps, small AVS. Getting there.itll just be a cruiser. My Reggie says 3675lbs. OE 318 auto buckets & console, AC, PB, PS, electric window car.
I have run the RPM Air Gap for a long time now. It is an excellent street, street strip intake. The regular RPM is heated, so it is a more flexible year round intake. Your cam choice. Someone can always come up with a better one and/or rank on what you put in. So long as it works with the compression ratio, gears, converter, you'll be fine.
IMO, there's only 2 headers to look at. TTI's or the 1-3/4 Hooker Super Comp headers which do not route under the steering linkage. Great starter & ground clearance.
I have run the KB-107's twice before. The new 360 for the wife's ride will have the 116 slugs. A 750 would do fine for the 360 & an 850 for the stroker. I'd stroke it if the funds are there.
3.55's are a bit low for the intended cam duration. I also wouldn't worry about a cast crank but for the price, strength and the abuse it will take, forged is the ticket. Can't argue that. But is it really needed?