Isolation is the name of the game. In the older Chevy trucks with a solid frame mounted steering column, 1/4" thick front cab mounts, a single front mount, and no transmission stick dampening, you were subject to an advanced "feel" for suspension bumps and engine vibration. Modern machines use a variety of isolators to keep vibrations to a minimum. Rag joints, fat body mounts, liquid filled engine mounts are just a few of the "improvements" that have come about in the past 50 years.
Personally, I want at least as good vibration isolation as was offered in the 50's. I've noted in my quest for making something that worked good even better, that the front mount clearances DID make a difference in the level of vibrations we feel in the cab. If you fail to establish at least the minimum clearances you will experience annoying vibrations.
The replacement rubber inserts that are offered appear to be slightly thinner than the originals. Checking to make sure that your front clearances are met or slightly exceeded will make your driving experience a good deal more pleasurable. I mentioned it in an earlier post and think it is noteworthy to repeat it again! If the 1/8" thick top floating plate that sits on the rubber mount, makes contact with the base, there will be a noticeable transfer of engine vibrations. If the rubber insert is installed upside down, there will be noticeable vibrations. If the large thru-bolt that attaches the mount to the motor plate isn't centered in the oversized holes in the cross member there will be a transfer of vibrations. Three very easy ways to diminish your driving experience. Three things that are easy to get right the first time, before you get everything buttoned up and are ready to cruise.
Admittedly, I found this out the hard way, and feel that an extra word of caution might be in order. Making corrections after your truck is on the road is not a simple proposition. If you take a little extra time to make sure that you understand the way the original system was designed to function and insure you meet the minimum clearances you'll have a much more pleasurable experience. I hope this helps.
Dave
