Let me throw my opinions at ya
I think what you are planning will work wonderfully...the vortecs are great, and 9.2-1 compression will work just fine on 89 octane with a mild cam...87 may be just a touch low if you use such a small cam though. 400's are a bit more likely to ping than a 350 due to the fact that they run a tad hotter (siamesed bores) and because of the larger bore size. With some really sharp tuning 87 will work...but it would take fine tuning of the advance curve and a top notch cooling system, plus you will have to pay very close attention to carb jetting. Your cam specs will also play as much of a role in this as the mechanical compression ratio. I run a 350 in my 78 chevy 1/2 ton truck that has 9.4-1 compression, vortec heads, headers, duals, performer rpm intake, 600 edelbrock carb, and a comp cams XE268 cam and kit (224/[email protected]
, .477/.480 lift on 110 lca/106 ica). I also have a 2000 rpm convertor and 3.08 gears. Vehicle weighs 4500 pounds. I can run 87 easily with no ping, but economy suffers slightly over 89 octane. I have gotten a best of 15.5 mpg at a 2600-2700 rpm cruise (65-68mph on the highway) and it runs [email protected]
I have way more wind resistance than you do. The cam may be slightly too big for my application...but it works. I pull my 23' camping trailer with ease, and from a rolling start I can beat a great many stock and mildly modified cars out there. I believe that with the tuning I have done since last year I can get very high 14's now (wont know for sure until I can get back to the track)...I really need 3.73 gears but I occasionally take long camping trips. The mph is a good indication of how well this combo runs...with 3.73's and traction I could have mid 14's easily. The vortec heads combustion chamber shape is so much more efficient than the older style heads...it allows more compression without detonation, and doesnt require as much spark advance either for max power. If you plan on keeping the 2.41's, then the 209/209 .414/.414 cam will work fine and be very torquey, but will run out of steam pretty early on a 400...but even 4000 rpm with 2.41's will be flying. For what you want, 2.73's coupled with the short tires you have would work great for economy and performance, but I would use a little bigger cam...something along the lines of the comp cams XE262. I would also use a dual pattern grind...its a proven fact that a mild dual pattern cam will get better economy with a street exhaust system and it also helps compensate for the slightly weak vortec exhaust port. Plus, this cam will work great with the stock vortec springs. The vortec springs are a little light but this also contributes to fuel economy...and since you wont be making power beyond about 5000 rpms then they will work great. Use a vortec pattern performer intake and a tuned q-jet, and I think with a stock convertor, 2.73 gears, small tube headers and duals, you could easily have a mid 14 second car that gets 16-18 mpg on the highway...traction will be a big problem with this much low end grunt though.
375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova