You bought it about 1 1/2 years ago?
Around that time there were several reports of people wiping out M4F cams (and possibly others).
At the time, I heard that some had used lifters made offshore, and that the hardness of the lifters were not correct. I believe I also heard that some of the cams hadn't been hardened properly either, but I can't confirm this.
I would hope that Patricks will help you straighten this issue out.
Technically, the M4F is practically the same as a stock 261 camshaft. I doubt you would ever tell the difference between them, especially with stock intake and exhaust. There's nothing wrong with using the M4F, but with the extra 26 cubes, you can step up to a slightly hotter cam and still be happy. Do not use the F298 unless you are going to do alot more to the engine.
Since the M4F is basically stock, I don't see why you would have to run stiffer than stock springs unless you wanted to.
If the head has been milled and/or the block decked more than around .020" total, then there should be a shim used under the rocker stands to compensate for the milling, otherwise it could add to your valve train geometry issues...
Did you use anything on the camshaft and lifters for break in?
Did you prime the oil pump?
What rpms were you running it at during initial break in?
Here is a page out of a 1955 shop manual on oil viscosities, etc...
http://pic20.picturetrail.com/VOL1205/4415295/9337405/175551634.jpg Check the card that came with the camshaft also.
