Advice about a 360.
actually i think you are doin' it right.
by having a complete core 360 you can build up, then swap in when ready, you are much better off. just doing bolt-ons usually is disappointing.
but with the 360 as a base, you can stroke to 408 swap in a solid roller save up for some decent heads, etc.
there are plenty of engine builders here. MRL comes to mind
contact them, i'm sure Mike will give you all the advice you could possibly need:
http://www.forabodiesonly.com/mopar/member.php?u=18781
http://www.forabodiesonly.com/mopar/showthread.php?t=241854