heads... exhaust... intake

With parts, it sounds right. Labor is $250 where I go (East coast) for 2 V-8 heads. But the parts..Valves*, guides*,seals, seats*,springs, retainers, and locks add up fast.
*-means if needed...
But if you read, I said "JY". Meaning go to a recycler, buy 2 heads and rockers with a gauranty of being not cracked. Then have the shop clean and pressure test them. It is fairly common for Magnum heads to have cracks that do not effect the cooling system or performance. So magnafluxing may show cracks. Pressure testing will tell you if they are good or not. If they pass, replace the springs with a set of performance ones that have the tension your cam needs, and you're good to go. Between the pressure test, the labor to swap springs, and drilling the intake holes in the heads for the LA intake shouldnt cost more than $200 depending on the spring costs. And those can be Chevy LS1 parts so they will be cheaper anyway. Then bolt them on using AMC lifters and the Magnum pushrods.