Too much cam and not enough compression or converter for this altitude with that setup. I've been in some 318's that flat out run even up here at 5300'. We've surprised many a big block with them even in a B body. You might be able to tame the cam some with Rhoads lifters and regain bottom end.

That said, I would look for a 360 magnum donor. There are plenty of those around the u-pull lots around here. No replacement for displacement, and you then have a foundation for a future build also. That will be plenty of fun and peg the grin-o-meter.