If you keep it you can still install a roller cam in it. The conversion parts are inexpensive, but a roller cam and lifters are probably double to triple the cost of a flat-tappet counterpart.
So you still have choices, but as you found, not really a choice to run the EFI setup unless you don't mind a LOT of work and modifications to make it fit.
And it's speed-density anyway if I'm not mistaken, and not a MAF engine so you have to be more cautious about your planned performance modifications I think.
Still a decent engine if you got a good price on it. But if you paid for a roller block, I'd still try to get one from them. The others would know, but I think everybody says the later roller blocks are better in other ways as well.
Good luck.
Oh, and for those that were talking about the other non-Cleveland possibilities, some identifications were backwards. The 351M is the "M" in the 335 engine family. The 400 is just a 400. No such thing as a 400M.
Been awhile since we talked about them so I wanted to throw that in there.
Paul