I have a similar set up. I was using a 400 block stroked to a 415 and used a real nice milidon pan. I had clearance problems too, but wanted the pan for clearance of the crank/ rods. So I used square washers for a long time that were about 1/8'' thick each and just stacked about 4 each side and used the rubber motor mounts that the metal on one side. The rubber was worn out and was one of the reason to use so many washers.
But then you end up hitting the trans tunnel/ fire wall. I forget the part number, but I got new ones from autozone and they were for a jeep.
Eventually I changed to a 350 block with standard stroke, so I went with the moroso Vega oil pan. Much better fit! the steering linkage can move without rubbing, and I'm not hitting the firewall / tunnel anymore. If you can afford the switch, I'd use the moroso pan, and maybe sell the milidon to someone local to help pay for it maybe.
The brackets for the alternator were just welded on to the motor mounts.
I also have a similar radiator. We are odd balls here, because you and I have the in and out on the same side which isnt like the majority of chevy's. You can run into issues with your cooling. If you could, I would suggest putting the alternator on the drivers side. Otherwise, I had to get creative with the radiator hose. The thermostat housing had to come the other direction and the alternator is in the way. I was having cooling problems untill I cut my hose and installed a filler so I could add water at the highest point.
Do you have a transmission cooler in the radiator? And what style fan are you planning on using?