I was going to recommend S&S headers but apparently they went out of business. My 351C 4v has a set of them in my 37 Ford. They might have worked they're kind of a mid length and made to clear a MII setup plus hugged in close to clear narrow frame rails.
If you want to move the engine back you can get a rear sump pan, I've seen an aftermarket option in Jegs, I bought a rear sump 400 4wd pan and pick up to get pan clearance.
Best of luck finding something, although I have to say its wrong to build a 351C with out 4v heads, jk.
69 Mach 1 428 CJ
Modern Upgrades-TKO600, FiTech, A/C