You're very welcome; this is always a tough problem with the LA 318 and the 2BBL 273: finding a way to cure the low compression and resulting low RPM bog, when putting in even a moderately large cam. I am not aware of any magic bullet, higher compression pistons either. You could deck the dickens off the top of the block but that costs $$ too (plus new pushrods). The piston change is the direct and clean path to get there. And yes on the .7% volume displacement increase; that is down 'in the noise'. As for the price of the hi-comp pistons, I am sure that part of that is market demand.
Buuuutt... one more thing (there's ALWAYS 'one more thing' right? LOL) that just popped into my wee brain: the KB's are a lot lighter than the stock cast pistons so you will have to re-balance the crank assembly. So that adds even more cost to the piston change, which may put the nail in the coffin of the KB pistons for you. A cam change or other changes may indeed be the way to go.
One thing to ask your machinist about is how much is will cost to mill about .040" off of you heads and the right amount off of the intake faces of the heads. (May require new pushrods.) The objective would be to get the chamber volume down to around 58 cc's Then your SCR/DCR will be 8.9/7.3 , and 9.2/7.55 with the 1121G gaskets. The latter is starting to edge in on decent DCR territory. (BTW, all the numbers I have given you are with your 270 cam installed do-to-dot, with ICL at 106.)
And yes, your machinist is right to mention detonation. When you start to approach a DCR of 8, then you need to be more careful with tuning and ignition timing. But I suspect you wold be careful about that. (How is your high frequency hearing, BTW? That is your built-in knock detector.) You are kinda between a rock and a hard place with that cam; you're gonna have to push up the DCR if you want to get rid of the weak low RPM performance. So it is either fix the DCR or change the cam.
If you would like to work these numbers all yourself just to play around with combinations, I use the Pat Kelley calculator. You can put in the piston/bore/head data to get SCR and then put in your cam data and rod/stroke data to get DCR. Once you get the various numbers into your brain (or written down), it is a snap to run new numbers to see where you are with any changes in parts or milling. The Pat Kelley calculator seems to give a bit more conservative (higher) DCR numbers than some others.