Stock bottom end 360 magnum

Looks like a clean core you have there. I have gone through lots of steps to do what you are looking to do with my Magnum. Unfortunately, with the stock pistons, you won't be able to put any sort of decent cam in and the compression ratio will hinder performance, especially with the stock heads. Maybe just throw the engine in as-is with MAYBE a cam and call it a day. Then save for heads, pistons, and bigger cam later.
I wound up spending over $4,000 on my "budget" 5.9L Magnum because it just didn't make sense to put any kind of money into a Magnum with stock pistons and heads. Instead, I went with Icon forged 10:1 pistons, cam, and Promaster aluminum 2.02" valve heads with roller rockers. This combo is still cheaper than most crate motors and should make a bit more HP than the 408 crate engines being sold today.