I just finished assembly on my new long block 460 and realized I forgot to install the oil pump shaft from the bottom side. Is this a major problem to install from the top? Any tips or advice welcome.

 

Shouldn't be an issue if you drop the shaft in from the top in the right position.
Edit: I don't think there is anything preventing you from doing this, but haven't done it personally. You should be able to drop it in. Rotate it counter clockwise only to prime the oil system

 

After some searching and some research, I decided to pull the pan and install the oil pump drive shaft the correct way. There is to much risk of it falling into the oil pan when taking the distributor out in the near or distant future.

 

Not sure what truck you are in, but you may not be able to drop the pan out of the way enough to get to the pump. This is because of the cross member in the way on most trucks.(maybe not 2wd) I would put a needle nose vise grip on the shaft to install it, but that's me.