Yeah, Calif Tags are determined on weight and value. So you keep the value component down on any part-time rig. So start with a decent pre-1989 (Calif diesel smog regs, v1) ex-ranch truck (usually lower miles) with a chassis generally configured as you wish. Run it in AG registration so you have more hauling options...
Do what you want to power/tranny. Cadillac Shutters, clutch fan, thick pipe exhaust w/ heat wrap, divorced stack brace, double pass muffler, etc. Run it over to commercial truck upholsterer's in Petaluma and get a nice interior with lots of padding and "fuzzy" cloth (like Dashmat) on the walls and overhead and it will be comfy.
We have so much stop and go traffic all over the state now, and my over-60 clutch leg is not all that happy after a while... I'm going Allison for sure. In a pinch, my wife could maneuver it if I was outside for some reason...
But no point in doing all that and putting a thrasher of an engine in it ... I'm not brand loyal, just has to make decent power (400+) to overcome the Allison and we're off.
The SS tanks are whole nuther can of worms ...