i'll play devil's advocate to this in the line of thinking of: how bad is the 318 he's starting with? i mean, just because he's got it doesn't mean that it's not rife with problems or needing machine work in much the same way a magnum would.
he's already established that there's a piston missing or messed up. so we know how far apart it is, so what else is missing and messed up? if the thing is gonna be worth a darn, hes gotta do pistons to bring up the compression ratio. so he's doing a bottom end. let's say he gets away easy from the machinist and all he needs is a next over, a clean up on the faces and for them to knock the cam bearings in and press fit the pistons.
he's mentioned that the heads he has basically need a full run thru, with parts likely, so there's another run thru the machinist's wringer. same as the magnum. or laying out cash for some useable heads... which is kind of a gamble, but not wild.
i guess what i'm saying is that there could be a lot of similarities betwixt the 318 and magnum in regards to machine work, which is something he can't do himself and a cost burden.
the benefit to the magnum, is that evidence has shown motors with reasonable mileage on them tend to fare better and can at times get away with just doing the bare minimum: rings, bearings, a dingle ball hone, and ship it.
it all hinges on the condition of the 318. if the bones is good, i'd go that route. cheap, easy, lots of parts, would come together quick without any additional supporting hardware. but if it's a tangled mess, the pendulum quickly swings back magnum.
but your point is well made. get it on the road as quickly and as cheaply as possible; then build a better motor on the slow boat. a fire kept stoked burns the hottest!