another point of reference, my 5th ave I have a mag headed 318, 9:1 comp, with a 259/259 adv, 208/208@.050, on 112LSA cam, A500, 27" tires and 3.55's....65mph is ~2000 RPM. my commute is 2 lane, with a somewhat hilly portion, doing 55 mph (~1800 RPM), I've never had it lug/surge or downshift.

same thing with my old motor, which was a stock long block 360 with a comp XE262. the old motor would surge a bit at 45mph in OD, the new one doesn't...

if it were my car feets, even if it is heavy, I'd go 3.55
s-3.73's, since you'll probably be running 28-30" tall tires, your 518, and a higher stall lockup (2800-3200RPM) to get the weight moving from a standstill.

for a cam, I'd probably run bullet's HR276/378 lobe on the intake, probably a 114 LSA, installed at 108, not sure on the exhaust, if you want more or less duration on the exhaust with a turbo car...I'd personally build the engine with an idle-5500 RPM powerband...