Isky E-4 solid has been around for years. I think at least one member is running such. Might be worth a look.
At some point you'll have to reconcile the "very hot cam" and 3.23 gears. The 284/.528 solid MP cam will certainly give you a rough idle, and that "coming up on the cam" feeling, especially in a little 273, but not much torque below 3000 RPM. Less cam would probably make for a quicker car that's a whole lot more pleasant to drive in traffic.
The cam you are thinking of is actually milder than a factory 340 cam, but a pretty reasonable choice. A Comp XE262 would be similar. Neither cam would be "very hot" by any stretch. Also, if you are thinking of stock 273 exhaust manifolds, there is little point in going much wilder with the cam. You won't make any more power, you'll just give up low end.
Why on earth would you downgrade to GM HEI?!?
