Level (side to side) the top of the dovetail as best you can. Then install the new picatinny base as mr. big described without disturbing the leveled action (i.e. don't introduce cant when installing the new base). I would suggest very gently tightening down the six screws in the order suggested by DIP in fairly small increments so it goes down fairly level. When you feel the screws are approaching the point where they will fully tighten up, start measuring again with the level and and adjust the screws as necessary to keep the base as level as possible, until they are fully tightened. If you just crank the screws down, chances are good the new mount will not be level with respect to the dovetail/action underneath. That might work ok given the relatively short distances you'll be firing a rimfire rifle, but it is possible to keep it fairly level with a little care during the installation process.
Once you have the base, scope, etc. installed, I would recommend seeing how the rifle shoots before doing anything else to it. My CZ455 Varmint Precision Trainer shoots reasonably well with Eley Match and Lapua Center-X. Only slightly behind those two is the Wolf Match Target ammo. Surprisingly, this rifle also really shoots well with Winchester Power-Point Copper Plated (1280 fps) 40 HP. All four of those ammunition types give 0.5" or slightly less (average) 5-shot groups at 50 yd. Surprisingly, the Win HV gives the best vertical and best overall precision at 100 yd, followed by Eley, then Lapua. Haven't tried the Wolf MT yet at 100 yd. My VPT came with the 24.8" slightly thinner contour barrel, so whether the ammunition preferences I listed above hold true for your Varmint (if it has the 0.866" barrel) can only be determined by actual testing.
In any event, how your rifle behaves once you determine its ammunition preferences will give you a better idea of what you might want to do next, if anything. If 0.4" to 0.5" A lighter trigger pull might be helpful to precision, so might a bedding job. You also may simply need to adjust the torque setting on your action screws. Apparently some 455s shoot optimally with it set anywhere from about 15 to as much as 35 in-kb of torque for optimal performance. Fortunately, it is very easy to start low and work it up to determine the optimal setting. Don't go too high because it is very easy to damage the aluminum pillars.
The factory bedding job on the Manners T4 stock on my rifle is not so great. The barrel is not straight in the channel, although it does not touch the inside as far as I can tell. Unfortunately, this rifle has a tendency to shoot very consistently for a number of shots, then suddenly start throwing shots either high or low. Often it can be by as much as 1.0 - 1.5" at 100 yd. The annoying thing is that it doesn't simply throw one high and then come back as it would if it were an odd velocity round. It will actually stay at the new POI (elevation-wise) for quite a few shots before eventually returning to the previous zero. This is not an ammunition velocity issue, even though that is a common issue with rimfire ammunition, because it does not correspond to velocity changes. I'm pretty sure it is a bedding issue, and I'm most likely going to have it re-barreled and bedded to kill two birds with the same stone. However, if yours shows more consistent behavior, it may not be necessary to have any extra work done to it. If you're going to swap out the trigger for a Mr. Fly, get on the mailing list for when CZ gets more in and check it several times a day. They had a new shipment of about 20, I think, in December. By the time I saw the notice, they had been sold out for several hours. They won't last long when the come in, so if you want one, act fast. A simple spring replacement/mod may be all it needs, but don't try to adjust the pull to low or it may be susceptible to slam firing.
As mr. big mentioned, I'm also using a .20 cal cleaning rod (Tipton carbon fiber 40"). The threads on this are different from the .22 caliber and larger cleaning rods, so I also bought the adapter. That way, I only bought a new .20 cal jag, but can still use the .22 cal brush/bore mop I already had. I also bought the Yodave Delrin plastic .22 cal bore guide. It fits in the 455 action, but you have to have it oriented in a specific direction that seems upside down to me. Anyhow, it works.
