I put a 350 in my 59 Suburban, (replaced the 235 6-cyl) and did not have any issues. My 58 2-ton has a 327 in it, no issues.
The only tight spot will be the HEI distributor cap being close to the firewall. You already have a small block V-8 in there, you are replacing with another small block with the same outer dimensions, just more cubic inches. You should not have any different clearances than you do now.
