I see you are in the BigBlock forum, but are talking Magnum
Additionally you will need a matching trans,rearend,and driveshaft,a V8 K-member or conversion mounts, a complete new exhaust, plus a HD suspension including; V8 springs, T-bars, HD shocks,and possibly some front steering repairs.
Plus V8 cooling system,V8 ignition, and some upgrades to the fuel and wiring systems, with possibly a new fuel tank and lines in your future.
Additionally; gauges
With the go-fast engine, stop-fast brakes are kindof mandatory.
And of course wheels and tires
In order for the Magnum to be peppy, $3000 is a good starting point. To bring the LA into the same arena, with new pistons and heads, it will cost about the same. If your Magnum bores are straight and round, you would be money ahead.
The LA's are a drop-in on the matching K and do not need the electric fuelpump,return line, nor a specialty intake. he Magnum may come with a serpentine belt system which has it's own hurdles to overcome.