Not a Tundra - but need some audio advice

So starting to do a little work to my ‘71 Chevy C30.

Truck itself is far from a show vehicle / but for 50+ years old now - not horrible either - just a straight old truck with a touch of surface rust and in need of a few spots fixes and new bed wood cosmetically.

While I have it down here doing a few body things - got thinking - why not do the radio. Again - no point in going overboard, just toss in one of the many retro fit radios and upgrade speakers while still am at it.

Budget of say 1500-2500 total. Would think for that I can get something that both sounds good and adds Bluetooth/XM. Not looking for blow out windows with volume/bass - but obviously adding an amp/sub would bring things up to 21st century standards

Doing a little research - appears a lot simply mount under the bench seat. Any reason why this is better than behind the seat (other than space saving) ? Guessing a couple of subs under and a decent speaker setup in the door panel would be a good starting point - anything else I need ?

Was going to stop by the guy who did my Tundra install for me to talk a bit further next week - but would help to have an idea on starting point before I do.

Once I get this done - then it’s back home to the shop for a new drivetrain in time for summer. Think I’m going 402 I picked up with the truck (currently was swapped to an 80s 350 years ago) - have transmission rebuilt and rebuilding the 70 402 that came with it with a few minor tweaks - should be a nice weekend driver. Won’t be overly pretty - but I’ve got a newer Tundra for that.