they should be a drop-in unit, but you should check with the manufacturer first. Normal, factory roller lifters generally have a yolk between the lifter pairs (looks like a double ended wrench) which holds the lifters from turning, and then a spider that bolts to the lifter valley with arms to hold each of the 8 yolks down. The bar you see connecting each pair of lifters does away with both of those.
Roller cam will cost you a bit more money in the department of valve gear, but not a whole lot if your RPM expectations aren't nutty (which in a big block I wouldn't assume they would be). The mass added by the heavy roller units on those lifters create a potential for valve float (not sure how much of this you already have figured out).
Last, but not least, you need to make sure those roller units are in good shape. Theoretically the units should really never wear (which is one of the points of having a roller cam to begin with) but it is possible that the motor was not oiled well or that the motor was revved and valve float had the lifters slapping against the cam (which damages the roller bearings). The last thing you want is to save 200 dollars on a valve train and a month later have roller lifter needle bearings all through your journals!
Those definetly dont look like big block parts,and something else to consider is that roller lifters are not something you should buy used.After time the needle bearings get flattened out and this will cause the lifters to eventually explode,and as pointed out,you dont want to be pulling your motor out to pick out all the little pieces!Been there,Done that!!