I always use the blue rubber end seals. I put a dab of Ultra Black RTV in the corners where the end gaskets meet the intake gaskets, put the rubber end seal down, and put another dab of RTV at the end seal to intake gasket meeting point. That's what the Fel-Pro instructions call for and it works like a charm.
'88 Thunderbird LX: 306, Performer heads, Comp 266HR cam, Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles. '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6