the engine was slanted originally to fit in ke10 engine bays, as it's quite a tall upright engine in one of them. that was the reason most engines in the 60s were slanted (chrysler/dodge slant six, etc). by then, toyota already had mass production tooling for manifolds and oil pickups and whatever, so it was more cost effective not to change it when the ke20 onwards got a bigger engine bay
the problem with the engine being slanted is that on long right hand corners (turn 1+2 of QR would be a great example) oil builds up in the right rear corner of the tappet cover, eventually starving the rest of the engine of oil (super bad), and potentially blowing out of the tappet cover all over your engine bay and exhaust manifold. i've had this happen to me, as has doug, as has stewart. it's a total smokeshow, not the good kind :D
the upright kit includes mounts and a new oil pump pickup, as the factory oil pump pickup is on a 30 degree slant just like the engine. i would also be inclined to grind out the casting dag next to the pushrods for increased oil flow, you should be doing this on a modified K motor anyway. it needs that more when the engine is slanted than upright
you also need to insert a wedge into the manifolds, so they go back to flat, and still seal against the heads
as nick said, then the gearbox is on a 30 degree angle and shifting is a little wierd, especially in 5th. i don't think a K-T bellhousing corrects for this either