sounds likely to be a 283, and in a 58 it just might be original - a 327 is just an 'overstressed' 283 and more finiky - the 283 will last longer, and can be built to be as fast as a 327
did you get the numbers off the block behind the head on the drivers side? [on flywheel flange - if it's in the truck use a mirror]
327=
[62-64]3782870, 3789817, 3852174
[65-67]3782870, 3789817, 3858174, 3892657
[68-69]3914660, 3914678, 3932386, 3955618
if it's outta the truck the crankshaft flange can indicate pre or post 68, and I think that at least the early 327's still had the filter can, maybe all of them
the number at the back will tell you
Bill
