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I am the new owner of a 1950 3100 that has had a 250 and TH350 installed. I am seeing that the transmission is touching the firewall in the tunnel. The truck is at the point where it has not been driven as it does not have accerlator or shifter hooked up. I am wondering if there is anyone out there with experience in this area that knows what I am headed for as far as vibration while driving if the transmission is touching firewall?

There should be NO contact between the engine and the body, or any other part of the running gear with the vehicle at rest, as there will always be a little movement of the drivetrain while in motion due to torque loading. Sometimes an engine/body/frame relationship looks good until a little torque is applied, and then something touches due to motor mount flex. This also applies to exhaust system components. You're going to need to re-engineer something, either sheet metal or motor mounts to provide adequate clearance not only while sitting still, but also some extra space to allow for torque. You'll experience a lot of vibration, and/or some loud banging, if anything gets together.
Jerry

Ok. You confirmed exactly what I was worried about. Now I need to consider best, most economical solutions you offered a few. I am hoping that someone that has put a Chevy 250 and th350 and has dealt with this issue chimes in. In the meantime, the sheet metal option: would this require the body be taken off? If so, this sounds major. The other option, lowering engine via motor mounts? Wondering if this would require new drive shaft and other stuff. Any more thoughts on these options?

During my evaluation of the interference today in the daylight, I can see light around the transmission and I should be able to persuade a 1/4” gap between sheet metal and transmission. Do you think a 1/4” of space is enough to account for torque loading?

No one can tell you if and how much vibration/noise there will be. By the time you find out, it will be too late. So clearance is needed now.

It all depends on where and how much as to what to do to fix. Need more info. What parts of the tunnel and what part of the tranny. I.E. Is it one bolt head or a large area? A pic or two would help to see if prying or cutting or bending or tilting or dropping or re-engineering is necessary. Minimum of 3/8 needed.

Check body mounts first and go from there.
I did have to trim about 1/2 inch, maybe 5/8" off the lip on my firewall with the automatic. I was lucky though. The tranny cover did not have to be trimmed....just the lower part of the toeboard.
If you have to trim it some no one will know with the floor mat over it......depends on how much you have to cut.

....have you checked pinion angle to see if the tranny needs to go...or down?

I like to have a finger thickness at all potential contact points and this is fine with new or good rubber in the mounts. For clearance checking we use Play-Doh as it does not stick to surfaces like clay. Once the interference points are found you can massage (not beat) in clearance with a ball peen hammer and if patience is used there will be a smooth contour and not a lumpy patch of tin. For most lowering the trans mount is easiest and as long as you maintain a crank center line to pinion angle of under 4* vibration won't be a problem.

Thanks everyone for the feedback. It’s so helpful and valuable to have experienced people offer insight. I have been able to massage the sheet metal in the firewall and form the transmission floor cover to match. I’m going to have so many other questions and appreciate the help.

Chevy made several different styles of rear transmission mounts; it may be possible to give yourself the minimal clearance by switching the rear mount. When I converted from a two-bolt mount to a one-stud mount on my TH350 it lowered my trans about 1/2-inch.

Also make sure there are no spacers in that rear mount that are elevating the engine. A previous owner could have been trying to adjust his driveline angle.

The 250 is about 3" longer than the original 235, so that is where your problem lies with whomever did the swap, might need to modify the core support to move the engine fwd, other options are also possible.

Ed

What condition are your cab mounts in? The cab mounts in my '53 were so bad the cab sat at least an inch and a half low in the front and the firewall rested on the back of the original 216. Engine vibration transferred throughout the cab was deafening.

If you've got some sag in your front cab mounts, correcting that might give you the clearance you need plus help with door alignment, etc.

You might be able to shim the cab and bed up an inch or so higher off the frame, but there's also the possibility of having to adjust the fit of the steering column to the firewall if you do that.
Jerry