The shop that did the porting should have a flow bench if they are claiming 300/230 cfm of flow, especially when using the stock 2.00/1.55" valves. Several well known LS1 speed part suppliers sell upgraded heads using LS1 castings and even though they are CNC ported (opposed to hand ported) and using larger than stock 2.02/1.57" valves, they don't reach 300/230 cfm's of flow @.600" of lift (296/220 cfm). I just hope you aren't a victim of a shop that thinks they don't need a flow bench to hit a certain flow number. Unfortunately visuals don't mean much in terms of flow.
The GM oil pan/pickup/windage tray options are '98-'02 F-Body LS1 or '05-'07 Y-Body LS2.
Although there are many years/makes/models that came with the T-56 and can be made to work with a S30/LSx/T-56 conversion, the easiest (and usually cheapest) options for doing a S30/LSx/T-56 conversion are the T-56's that came from the '98-'02 F-Body w/LS1 or '04-'06 GTO w/LS1 or LS2.
Lastly, buying a pre-engineered kit is the way to go. Like you basically said, why re-invent the wheel? Plus building your own mounting kit might not work with the JTR headers you just bought.
S30/LSx mounting kit options are easy since JTR does NOT make a mounting kit for the LSx series of engines, just GEN I and II SBC engines (its a common mistake since they sell Sanderson LSx swap headers intended for use with JCI's S30/LSx conversion mounting kit). JCI is the only company that produces S30/LSx mounting kit that I am aware of.