magnums started in 92-93
they are a roller cam block,they have the closed chambed heads that have pedistal rockers and oil up thru the lifters and push rods.
there is a slight differance in the balance of the crank/rods/pistons
it has higher comp ratio than any of the older LA engines.
the heads are about the best flowing heads for a SB now days and are swaped on to some older LA block with some mods to make a pre-mag hybred engine.
85 started the LA engine with a roller cam and non oil thru lifters and shaft mounted rockers 89 for the roller block pre-mag 360 engines
they went to oil thru roller lifters and hollow push rods oiling the shaft mount rockers along with it oiling the old LA style thru the block/heads...oils both ways in 1990 or so.
more gain with the mag blocks as it is already for roller cam upgades
with out machine work the mag heads dont take much lift but can be worked for anything in the retro roller cam line up.
my LA 88-4-v 318 has 360 heads but as all 4-v engines had a flat tappet cam,I used all 1990 oil thru roller gear and hardware to swap in a lunati voodoo roller cam with some comp 901-16 springs
next up grade will be a set of mag heads for the LA/magnum hybred combo,it is ready to oil thru the lifters and PR for the mag head swap
the magnums have a short nose cam and no mech fuel pump,but you can get an extintion to run one with an LA t-cover.
the old LA t-cover will swap to the mag engine,heads may not have correct bolt holes for alt brackets IIRC
intake bolt angle is differant and can be redrill to LA bolt pattern to up the intake choise.
just a few things I remeber right off hand
hope that helps some