Only drawback to the complete engine is you are almost guaranteed to get cracked heads, granted the cracks most often aren't fatal. Aftermarket heads are $500-600 a pair.
What is so hard for the magnum topend upgrade?
You use Magnum head bolts
You use an aftermarket lifter for a MOPAR (the ones you buy over counter already are oil through)
You use 7.625-7.650 hollow pushrods ($48 at Summit)
You use stock Magnum rockers
You open the pushrod holes a little in the heads
http://www.hotrod.com/articles/mopp-0308-adding-magnum-to-a-360-short-block/
He could always just go with a mid to late 80's 318 head ( the 302's) too, they are swirl heads. I think they would be a bolt on but not sure if they are worth it. Would need a small port intake though, if his has larger ports.
I didn't realize the 70's LA had the same engine mount brackets as the magnum - more of a big block guy until I had a 5.9 in my 01 dakota.
I wish you could convince my 16 hemi w/8 speed that the hemi was more fuel efficient than the 5.9 - almost identical to my 01 5.9 dakota and there is only a #500 weight difference between them.