The Comp Cams 280 will need alot of initial timing, which the engine likes, & that is what you have found with the 40 degrees initial. Some of the aftermarket vac adv units add 30 crank degrees of advance. If the mechanical advance of 30 degrees comes in too early because of too light springs controlling the weights, then you could see around 75 degrees at idle, which would be higher than stock with that 280 cam. My suggestion would be to advance the cam 4 crank degrees, which will improve the low end a little. It may be enough to give you the low end improvement you are after. Rhoads lifters will also improve low end, probably more than advancing the cam. You should check intake valve-to-piston clearance at TDC & just after TDC because advancing the cam reduces this clearance, although you should be ok with 670 heads. I would also check out the rate of advance of your mechanical advance to see if it is coming too quickly.
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