What are the available solutions for a 8-input OR function that is 74xx compatible?
You seem to have covered most of the obvious ones, but you are limited by the 5V supply and whatever you mean by compatible with 7400 series (which could require the thresholds be TTL compatible). 

If I assume the inputs have to be true TTL compatible, I would suggest a 74ACT02 quad NOR gate (6.5ns) and 1/2 a 74LV20 4-input NAND gate (6ns and 12mA drive) so total 12.5ns, which is well within your spec.

Generally the gates with 4 or fewer inputs seem to be more popular, so the range of types and availability tends to be higher. Of course you could use a CPLD, which is where a lot of the logic gate market has disappeared.