Your math on the offset is correct, if you wanted to go from a 17x8 +30 to an 18x9 without moving the outer face of the rim at all you'd need an 18x9 +43.
That's not going to work though, because an 18x9 +43 works out to 6.7" of backspace, and if your rear springs are in the stock A-body location with that '67 B-body 8 3/4 you probably have about 6.8" from the wheel mounting surface back to the springs. So, no room for tire or clearance with a +43 18x9. And if you've got standard 73+ disks up front an 18x9 +43 won't work there either.
But you're not sunk yet. Here's the thing, your 245/45/17 has a taller sidewall. That taller sidewall means the tire hangs out further past the lip of the rim than it will on an 18" rim. That's why if you look at 17" and 18" tires a 245/45/17 will fit on a 7.5" to 9" wide rim. But a 245/40/18 needs an 8" to 9.5" rim, even though the 17" version has a section width around 9.6" and the 18" is at 9.8". By the section width you're only adding .1" to each side. So, the rim itself will stick out further, but the section width of the tire, which is where your clearance issues will be, will stay almost the same, with only an extra tenth or so added to each side and the centerline in the same spot.
So, if I were you I'd still be looking for an 18x9 +30 to run with a 245/40/18. Especially in the back, because I don't think you have much room to give to the springs if they're in the stock locations. Like, none actually, if your WMS to spring is really ~6.8". You honestly might need less offset in the back, since the 18x9 +30 will be at 6.18" for backspace, leaving only .62" for clearance to the spring (assuming you really do have 6.8" there) and some of that will be tire, say .25", which puts you at about 3/8" for tire clearance. I've run that tight with 18's without issues, there's really not much sidewall flex so it can be done but that is as tight as you can go. As for the front, I run 18x9's with an effective +30 offset with a 275/35/18. That's quite a bit more tire than you'll have with a 245/40/18. When I originally started running those I hadn't rolled my fenders, only trimmed the lower corner and extended the bumper to lower fender brace. But that was all for turning clearance with the wide 275's. You could run more offset in the front, a +35 or maybe even a +38 would probably work with the 18x9's and 245's. But I'd be inclined to stay with a +30.
