Well if you have a running flat tappet engine in the truck it's easier to convert it to a roller cam since you wouldn't have to pull the engine. But the other side of the coin is the ~$1,000 or so it will cost for the small base circle cam, lifters with the linking bars, new pushrods, etc.
If you're rebuilding the engine anyway get the roller block. IIRC, the blocks were roller ready after 1990 but they didn't start installing factory roller cams until 4-5 years later. The spider, dog bones, and lifters are easy enough to find on eBay if it's a factory roller block with a flat tappet cam.
The 351w wasn't roller ready until 1994(F4TE casting number).. and was found in all F and E series trucks.
Converting an older block to roller isn't really cost effective, the linkbar roller lifters required to run a standard roller cam will cost at least $350, and then you also need pushrods and most cams run $250 or more. You can install the factory roller assembly(spider and dogbones) if the engine is being torn down for a rebuild..the valley has to be drilled and tapped for the spider retainer, but then you have to run a small base circle cam which will cost at least as much as the regular roller and there really isn't much selection. Compare all that to a flat tappet cam and lifter kit which will deliver the same power for as little as $100.
Good luck finding a roller block for cheap, i have been looking for one for my 93 cobra for some time now and cant seem to find one and the few i have found the person knows what they got and jacks the price up
You just have to search constantly and snap up the engine before the next fellow does. I got one for $200 after about a year of searching parted out vehicle ads, not even visiting wreckers. However, scouring junkyards should be successful.
Also look for these engines in 1994-up heavy vans.
Didn't the Cobra 351 have a unique casting number block or head that was otherwise identical to other castings? I seem to have read about that as one fellow in California had to find the correct casting number Cobra part to satisfy the pollution authorities. He was having no luck.
It would be next to impossible for me to find a 351w block out of a 95 cobra R, i was just looking for one to put in my 93 cobra after i pulled and stored my original engine definatly making shure i could put everything back to original when i needed