Doing same swap 305/350 in 53 GMC Longbed. My plans are to cut center rivet crossmember but leave ends on frame , use center motor mount on engine and new rear trans crossmember. I have a mustang 11 ifs and it looks like side mounts will be partially welded to new ifs crossmember. has anyone done this? Would like to move back to avoid cross member also when you refer to cutting toe board do you mean the lip on bottom of cowl. Thanks