100% times 2 on cleaning the lifters. I just did this with swapping a 5.9 into my 66 Barracuda. the amount of crap inside the lifters was mind blowing. my 5.9 was just as sludgey as yours is, but an hour or so of soaking in Varsol (parts Cleaner) and then taking the retainer clip out and the plunger of each lifter and cleaning them out with tiny brushes did the trick. After getting mine all back together, not a single valve train tick. plus way cheaper than a coupe hundred bucks for new lifters. just be careful not to shoot the retainer clips across the room!