Moving 1972 4x4 engine crossmember

[Chevymaxx]

I am going to be putting a 454 in my 72 k10 and from researching the best way sounds like moving the engine crossmember forward the the next holes. Here’s the question, when I try to slide it forward I can’t get more than an inch before the frame narrows and the crossmember gets held up hitting the sides. Is this normal? Everything I’ve read on doing this never mentions narrowing the crossmember to accomplish this. Also the only “forward holes” on my frame are on the drivers side top. Is that also normal? I understand that I need to drill some holes but am I missing somthing? Thanks for any help.