*351W - ~400 Engine hp
*Edelbrock Performer RPM Top End Kit - I'm not 100% on this one. Was told by PO it has top end kit with ported/polished heads. Intake is Performer RPM.
*Holley 670 Street Avenger - tuned well
*Bored .30 over
*2 1/4" exhaust
*4 Speed Toploader - Not really interested in a 5 speed just to get overdrive. I like my toploader
*3.55 Detroit TrueTrac - gear swap to feel like I have more hp/torque? Rarely am I on the roads doing more than 55 mph.
*Car has old school traction bars and no issues with wheel hop
*Flex fan - never had any overheating problems even in 95 degree weather. Flex fan can be annoying though.
*Front disc - brakes manual - I'm fine with it
*Manual steering - I'm fine with it
MAybe have it dyno'd to verify HP numbers and from there you could retune or see what power adders you could do. 400 hp seems pretty good though...a gear swap might get you the seat in the pants feel of more power- but I am not sure if you went higher (gear) with 400hp if you would be able to "hook" up or just spin all the way down the road (but you said you like to spin tires) How about a line lock for the brakes so you can really spin?
The first thing I thought of was higher gear, but had to re-read to see the 3.55 TrueTrac. I know you're looking for suggestions, but it sounds like the car is doing EXACTLY what you want it to do. Maybe a safety feature or two, like upgraded belts, etc...