How far in the hole 318 pistons

Furthermore
The factory put these together with 020 steel shim gaskets, with a bore hole suitable for a 318. I can't say exactly how many ccs those were, but I would think that ~4ccs would not be far off.
The thinnest I find now are 028s and based of a 340 hole, making them about 6.8cc.
The factory cam, installed straight up, has an Ica of, IIRC, of 48* and gives a CCR of about 140 psi at sealevel.
If you are at a higher elevation you WILL lose pressure.
If you install a cam with a later intake closing event, you WILL lose pressure.
If you give your heads a valve job, you will probably lose pressure from stock.
Pressure is heat is power.
Losing pressure will make the low-speed operation, and throttle response softer. At that point you will need a higher stall convertor to get off the line, with the same authority as what the stock cam gave you, and even more stall to make it more fun. and
forget any factory 318 gearing, Most cammed-up low-compression 318s are gonna need 3.91s . and
don't forget the headers. a bigger cam needs headers to shine. They work thru the overlap cycle, helping to get the A/F charge in the plenum moving. With the pistons down in the hole at TDC and dwelling there, this slows down and weakens the draw on the plenum, and the timing of the event takes a hit, nearly negating the whole event. Throw in the fact that most guys can't get the ignition events on time, and the whole idea of making power with an 8/1 318 just falls apart. and besides all that,
the factory cam already has timing events are nearly ideally suited for the low-stall TC, the hiway gears, and the log manifolds.
all that engine really needs is a higher stall, a 4bbl, better gears, and a free-flowing dual-exhaust system.

Now I heard you say "custom cam", but again; at your pressure level, every 4* later ICA is worth about 3>4psi, and the factory 140 psi is already super low.
for example;
the 340 cam has an Ica of 64*, compared to the 318s 48*, and with that cam, the pressure is likely to be down to 122psi versus the stock 140, both at sealevel. Thats an 18 psi loss for 16 degrees, equals 3.55 psi per 4 degrees. You think 140 psi is sucked out, wait til you drive 122psi!
That 340 cam wants a true 10/1 at sealevel to get the pressure up to 163psi, close to the limit for best gas in Canada, 91, (R+M)/2, and that would be best case.
Therefore
I highly recommend to get your pressure up to match your cam, or
just install a 2800 convertor and performance 3series gears tomorrow, and forget the cam. and, with a 4bbl you'll want a free-flowing exhaust, and I would slap on some cheap headers for good measure.