According to the received wisdom, "there is no substitute for cubic inches." That is why 270s and 302s are "all" you "see." Torque in the low and mid range is the great strength of these engines. Torsional vibration will not allow them to live at high rpm. Cubic inches make torque.
A 270H or 302 head (with 302-style pistons) would considerably increase the breathing capacity of your 248, probably at some sacrifice at low rpm and relatively little concomitant increase in rpm potential at the top end.
An interesting alternative to consider is a 302 block and head--or a 12-port head--with a 248 crank. The shorter stroke would give you an "oversquare" engine without a great loss in displacement. Indeed a 4.100 bore with the 248 crank (3.8125) will yield 301.6 cid, almost exactly the same as a stock 4x4 302. i have thought that the potential gain in sustainable maximum rpm with this combination would make the most of the improved breathing of a flow-through 12-port head.
For a street engine, your 248 will respond to the same basic tricks that we apply to a Chevy 235 or 261, as well as to a 270 or 302. You have 13 more cid than the small Chevy, but you start 22 to 54 cid down from the bigger GMCs. i think you would find that a 270 with 270H head or a 302 would give you the best performance for the time and the money in street driving. Yet if you are able to make the 248 perform really well, everyone will admire it--even those who will say, "Why didn't you start with a 302?"
God's Peace to you.
d
