amp between the seats is a bit sucky..... going by the showroom thread photos.... what about inside the glove box and up a bit ? so its above all the junk in there ?
(my brother used to have this setup in his van.... with a 6kw amp
as for speaker postions.... stick them in the old holes if you can, or wherever they fit best... unless youre really big into your sound quality etc... you want a van with a radio in it, not a sound system with a van to go with it
also... make/buy a box for the amp to go into, and fix it to the roof in the back, not sure how to make it detachable tho (and still look ok).... (this is apparently the best position for them anyway)