Exhaust Port Config of LA vs Magnum Head

Flange yes, but there was some header fitment issues with some of the aftermarket iron Magnum heads.
(IIRC)

Only thing I can think of is the ports dont match exatly- IIRC the magnum style port exhaust floor sits higher than the header floor, but it still seals. Haven't run into problems with EQ's or RHS heads with header fitment.