Buying a new engine, opinions?

I guess it depends on how you intend to drive the car. A 351 4V will certainly be potent and fun to drive, and an easy install. A big block would probably be a better choice if you are limiting your use to the quarter mile drag strip.

IMO you should consider stroking the 351 before going to a big block. That way you can still turn a corner here and there. Especially with the 4V heads, I would think it would make loads of power. Then you might be able to keep the C4 as well.

I'm in a similar situation, my car was a 2V/FMX car originally... My original plan was to build a stout 501 long rod and put it in there, and am well on the way to building it but have had some things come up that require me to re-evalute how much I can invest total on the car. At this point, I think I'm gonna sell off my RPM heads and my Eagle crank and a few other parts for my 501, since I have a good 351C block, accessories and pulleys, mounts, etc, and it would be cheaper overall for me to build a high-winding 4V/Toploader instead of a 501/C6. The little things for a big block swap are really the killer, otherwise I'd still be all for it.