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.