Since you already have heads, I broke it down into HO and NON-HO. Not that you'd want to re-use a stock cam.
ROLLER 5.0 / 302 ENGINES WITH H.O. / 351W FIRING ORDER
Firing order: 1-3-7-2-6-5-4-8
Make Model / Vehicle Years Engine Type Notes
Ford Mustang 5.0 H.O. 1985-1995 5.0 roller cam, H.O. firing order 1985 manual-trans H.O. was carbureted but roller-cam. 1986-1995 EFI H.O. engines use this firing order.
Mercury Capri 5.0 H.O. 1985-1986 5.0 roller cam, H.O. firing order 1985 manual-trans H.O. was carbureted. 1986 EFI Capri 5.0 follows Mustang-style H.O. layout.
Lincoln Mark VII LSC 5.0 H.O. 1986-1992 5.0 roller cam, H.O. firing order Good non-Mustang donor family. Verify accessories/intake differences.
Ford Thunderbird 5.0 H.O. 1991-1993 5.0 roller cam, H.O. firing order MN12 Thunderbird 5.0 H.O.
Mercury Cougar 5.0 H.O. 1991-1993 5.0 roller cam, H.O. firing order MN12 Cougar 5.0 H.O.
Ford F-150 / F-250 light-duty 5.0 1994-1996 5.0 roller cam, H.O./351W firing order Truck 5.0, not “Mustang H.O.” output, but uses the 1-3-7-2-6-5-4-8 cam/firing order.
Ford Bronco 5.0 1994-1996 5.0 roller cam, H.O./351W firing order Same basic late truck 5.0 family.
Ford E-Series / Econoline 5.0 1994-1996 5.0 roller cam, H.O./351W firing order Late van 5.0 roller family. Verify donor because vans can be mixed-use/fleet oddballs.
Ford Explorer 5.0 1996-2001 5.0 roller cam, H.O./351W firing order Excellent donor. GT40 heads early, GT40P heads later. Distributorless Explorer-specific front dress/intake/accessory differences.
Mercury Mountaineer 5.0 1997-2001 5.0 roller cam, H.O./351W firing order Explorer sibling. Same general 5.0 roller/GT40/GT40P donor family.
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ROLLER 5.0 / 302 ENGINES WITH NON-H.O. / STANDARD 302 FIRING ORDER
Firing order: 1-5-4-2-6-3-7-8
Make Model / Vehicle Years Engine Type Notes
Ford LTD Crown Victoria 5.0 1986-1991 5.0 roller cam, standard 302 firing order Panther-platform non-H.O. roller 5.0. Not a Mustang H.O. cam/firing-order engine.
Mercury Grand Marquis 5.0 1986-1991 5.0 roller cam, standard 302 firing order Panther-platform non-H.O. roller 5.0. Uses smaller/non-H.O. supporting parts.
Lincoln Town Car 5.0 1986-1990 5.0 roller cam, standard 302 firing order Last 5.0 Town Car years before the 4.6 modular. Non-H.O. roller 5.0.
Ford F-Series truck 5.0 Approx. 1992-1993 5.0 roller cam, standard 302 firing order Transition-era truck roller 5.0. Verify before buying; trucks are where year/application confusion happens.
Ford Bronco 5.0 Approx. 1992-1993 5.0 roller cam, standard 302 firing order Transition-era truck roller 5.0. Verify actual cam/firing order.
Ford E-Series / Econoline 5.0 Approx. 1992-1993 5.0 roller cam, standard 302 firing order Transition-era van 5.0. Verify donor carefully, especially fleet vehicles.
Best donor shorthand:
1986-1995 Mustang 5.0 H.O., 1986-1992 Mark VII LSC, 1991-1993 Thunderbird/Cougar 5.0 H.O., 1994-1996 F-series/Bronco/E-series 5.0, and 1996-2001 Explorer / 1997-2001 Mountaineer 5.0 are the common factory roller 5.0 donors that already use the H.O. / 351W firing order.
Be careful with 1986-1991 Panther cars and early-1990s trucks/vans. Many are roller 5.0 engines, but they may use the old 302 firing order.