700r4 swap crossmember questions

[Low84]

I have an 86 c10 305/th350 I'm swapping out in favor of a 350/700r4.

I was told to just move the stock crossmember back and drill the holes in its new position. I'm taking that advice as I can't anything else that involves not drilling into the chassis anyway.

My question to you guys that have done this before is, did you have to drop the gas tank in order for the crossmemeber to go in the right place?

I figure I have to anyway since I need the room to drill, but I don't know if the crossmember drivers side portion that goes under and up the side of the chassis will cause problems or rub against the tank down the road