The Small block d50 swap requires a Car oil pan, and chances are the oil pan will sit tight against the crossmember. The only other option is to modify a Truck oil pan (convert it from a rear sump to a front sump)
And the firewall needs to be modified or hammered back to clear the distributor
My d50 has a different frame under it, so I didn't have that problem with the oil pan.
It's easier to shove a B/RB in a D50 than it is to put a Small block in one, both require core support modifications, but the big block doesn't require firewall modifications, and the oil filter isn't in the way of the exhaust either. You thought routing the exhaust on an A-body was bad? try doing the exhaust on a Small Block D50, I had 2 days worth of work getting the exhaust routed back past the transmission.
No small block headers fit either, you're stuck with exhaust manifolds. Sometimes the driver's side truck manifold works, sometimes you need a 340 or early a 273 manifold. I slid my 273 to the left and tilted it at an angle to get the oil filter to fit (you can't run a 90* adapter, there isn't enough room) and to run truck manifolds.