318 Magnum pistons and rods in an LA motor

the 3751841 crank was used for MANY years, from at least the 80s thru the end of Magnums; I have used "LA" cranks in Magnums and "Magnum" cranks in LAs;
at one point I had an 88 D-100 w/a 318 and I went to Car Quest and priced a crank kit; IIRC the interchange was like '68-97; (this was back in like 1998) so as long as you used the whole set yes, Magnum or LA rods CAN be interchanged, on 318s.

the "different balance" on LA vs Magnum referenced above is primarily related to 360s which were external balanced. ALL 318s were internal balanced (from the factory anyhow)