What motor to go with?

if want to build up the 318 like you say..then is sounds like you would be good with a comp 268* hyd cam, the hyperutectic KB 167 pistons and some regular ceviite77 tri metal bearings w/ some single moly rings, stock pan/oil pump...'maybe' windage tray

The heads need to be considered 'not really for what you're asking' but lets look at them anyways..

stock '675' 318 1.78 valve...........stock '974' 360 1.88 valve
--------int cfm-------------------------------int cfm--------
.100--53---------------------------------------55
.200--107-------------------------------------109
.300--154-------------------------------------169
.400--159-------------------------------------206
.450--162-------------------------------------213
.500--165-------------------------------------208

The 318 heads 'except '#302' have chamber that are around 68cc
The 360 heads all typically have around 72cc chambers

ALL factory la 318 heads have 124-125 cc intake ports

1.88 valve 360 heads have 160cc intake ports

With porting being considered...
The 318 heads pored will go 200 cfm fairly easy with a stock valve, more like 206-210cfm with a good valve job, they tend to peak around 240cfm on an average.
The 1.88 valve 360 heads ported will go 240-250 fairly easy with a good valve job, see here http://www.forabodiesonly.com/mopar/showthread.php?t=145748&highlight=360+head+trip

You could just keep it simple and with the fore mentioned bottom end build and 'to keep it simple' run a set of milled 1.88 valve 360 heads, they're worth another 40 cfm over the heads you have now, cost a lot less than porting AND building the 318 heads you have.

jmo