Built many trucks and all had 410's. My choice in the same truck was a completely stock 69 340 with the AVS it came with. It pulled like a freight train.
I had a 73 360 HP in one and I did not like it. The only other choice would be a P pump 5.9 or 2004-2007 5.9 Common rail. I did that too the problem was the cost. I also did a Dyers blower on a 340, The common rail out performed it. You ask for an economic advice Look for a 10.5 340 use iron heads and dual plain intake.
" Or keep your 318 and install early 340 large valve top end and cam". This would be the best bang for the buck. Now this is my opinion on what worked for me using what I had, I am sure there are late magnums out there that would work just fine with fuel injection for today's fuel.