Well, even though the Blade rules only cover technology to about the Thirty Years’ War I actually don’t see a problem concerning the firearms, at least not for the 18th century. Breech-loading guns, vastly improving reloading times, were to my knowledge only starting to become common in the first half of the 19th century, as was rifling of the barrels, improving accuracy especially over longer ranges. And even though pre-measured paper carttridges for muzzle loaders were in use in the 18th century, those were no real cartridges in the moderns sense and did not speed up reloading times so much as to put the rate of fire of firearms into an entirely different league than older missile weapons. I think you can just use the weapon stats from p346.
You would however have to draw up stats for the smallsword, which would be the most common serious weapon encountered in a fighting man’s hands in 18th century London.
Fright mechanics did of course not have any place in a game about Sword & Sorcery heroes, but a few ideas to consider for your game:
For a game of horror I suggest spreading sanity and fear mechanics out among several Attributes, so as to prevent players from beefing up on one to become completely unfazeable. SY might govern how much somebody can take, being some kind of reservoir of sanity that gets chipped away over time; sanity might initially equal SY. TY might be considered one’s capacity to keep it together – a TY Check might be used to see whether any sanity is eroded in the face of deeply unsettling revelations or especially gruesome events. Mere fright might be dealt with by DG, which covers bravery and pluck. While a failed DG Check should probably not erode sanity, it might result in a PC being unable to go on, or to at least suffer from increased TNs (until an immediately life-threatening situation’s rush of adrenaline clears his system for at least the time to be able to deal with the threat by means of either fight of or flight).