Looking for a little more power out of a 318
Robbie has a point. But you cold probably do well in the HP department without to much work. I don't know about 375 HP.
Try this, one of 2 ways, on the cheap or one with some extra dough for new parts;
Mill 318 heads down for a better ratio, 9.0-1 would be the min. ratio and good enough to use. Port to match 360 head w/1.88 valves back cut and continue with a bowl porting, add small primary T-Q w/headers. Cam speced on or about 224 intake duration @ .050 and more on the exhaust.
An excellent example is the Comp XE268, though I'd like more lift myself.
You'll need about this much cam at a min. to make the 350 HP mark with this set up.
For more power and a bit more on the price tag without piston replacement, try a Comp XE274H or the Magnum series280H cam and a Magnum head conversion, Edel. RPM (Magnum style intake) 750 Carb of choice. Edel. or Holley etc.....Headers
Remember the new rocker ratio gives new and higher lift.
This should get you to 375 HP and better with proper tuning.
Thats the HP soulution IMO and you'l have some mileage, but not like you have now. For a well set up car combo, your going to need a stall converter to match and some gear back there.
I would also look into a 4spd auto. trans like the A-500, a 904 based overdrive. You could run 3.91's in the rear while, with the overdrive, the final ratio would be 2.6979.