Not knowing your budget, I'd say port the heads if they aren't already, change to a nice solid FT cam, my have to change rockers...KB pistons and zero decked to get the compression around 10:1, rebalance the crank, and change the carb to a 1000HP.
I did a 413 for a customer that we bored the block to 4.250 to make it a 426cid, ported Source heads, nice 275HL comp hyd. cam, 9.5:1, 950HP carb and 1 3/4 headers.
It made 480HP and idled nice with a slight chop.
It was a very mild build...