The two issues are oil control and the tie bar clearance. I will not use a roller cam without bushing the lifter bores. Period. However tubing the block will have some of the same affect in terms of oil controll. The issue is leakage around the lifter at max lift or on the base circle, depending on lfiter design and cam grinder. Also, should the valve train fail (much more likely using a roller cam and associated support stuff) then the engine looses oil pressure to the mains due to the oiling system design. So you should do something there. The other issue is the tie bar clearance around the factory block. Comp makes a good set that sometimes drops in, sometimes needs some grinding, but minor stuf. However, Brian (OU812 on Moparts) has a custom set from Comp that drops in and uses a better pushrod seat angle.