J (915) vs. Magnum heads

Figure on completely rebuilding the Js if they at the point of needing a valve job. Valves, guides, springs, locks, retainers, valve job, milling to equal out the compression loss, ect. Not to mention needing the LA valve covers, rockers, shafts/hold downs, head bolts, intake, ect.
Even once this is all done, you still need a fair amount of port work to get the Js to out flow the Magnums. Just swapping the Magnums for the Js in near stock form would result in a performance loss.