We solved this on an S-10 by just modifying the pan. Slant the front edge of the sump. Cut that entire front edge of the sump across from one side to the other at 30° angle to clear the crossmember and weld a new piece in, maybe an hours work with the pan off. It was the easiest fix, it was a similar Moroso pan that had a screen, scraper and trap doors built in, there was no way we weren't going to use it, a stock pan wasn't even considered.
If you have a welder, the crossmember cut and box is also a very viable option, racers do it alot to fit even bigger pans than you have.
With the motor the looks of what you have performance-wise, I would not even consider a stock pan, especially with a strokers longer stroke/bigger counterweights reaching down into the pan and pulling the oil up out(bad windage).
Summit also told us the pan would fit, no problem!! Catalog says it fits all GM rear drive except Vega, '62-67 Chevy II, and Chevelles with steering linkage behind the crossmember(duh, which don't exist
) Ha Ha.
I don't think you will gain much clearance to move the engine forward with a stock pan either, maybe 3/4" or so. You don't have a lot of room up front for radiator and water pump even with a short pump, and moving the engine forward may also cause problems with header fit.