Installing a 348/409 in a TriFive is not as difficult as many proclaim.
The 348/409 will bolt up to the trip-Five original V8 Bellhousing. This bell housing provides the rear engine/trans mount as well. You can install an 11" clutch in this Bellhousing.
The fit between the #5 Exhaust Manifold exit and the Steering box is easily remidied by grinding off 3/16" from the contact point on the Manifold, and 1/8" off of the side of the adjacent Stock steering box. There is PLENTY of meat there to grind off to result in a 1/4" + clearance. Unless you have a non-stock steering box with a U-Joint, don't off-set or otherwise try to move the box, because the stock box needs to go arrow straight through the firewall to align properly.
You can use the Stock Chevy Motor Mounts in the front. The best way to do this is to very, veryslightly suspend the engine with a lift, transmission mounted to the bellhousing and bellhousing bolted to its Frame mounts. With the front mounts bolted to the engine, lower the engine to just touch the front crossmember. Where the mount contacts the crossmember should be about 3/4" to 1" forward of the Stock V8 mounting holes. Mark where the center of the mount touches the crossmember. Make sure the markings are even, centered and symetrical
Now drill the holes (2), then use a taper drill to finish the hole to the correct size (Same diameter as the other two, V8, and 6 cyl. mounting holes. Install the mounts on the engine and crossmember, and that part is done.
The Oil Pan will need to be modified or replaced because the front of the reservoir section interferes with the steering linkage. If you cut out the front section for clearance, realize that you are substracting oil capacity, and need to replace it by extending the remaining reservoir downwards. Roughly a 1.75 inches downward extension gives you a 5 1/2 quart oil pan, which is perfect for the "W" series engine. If you are not a good welder, take it to a pro, explain what you are trying to do, and they can weld it up for you. I used a Wire feed welder, and it was easy. Check for leaks with gasoline or acetone, not oil or water.
A modern three row Aluminum Radiator, mounted in the 6 cyl position works perfect, but also be sure to add a shroud, and a 6 bladed fan. These are available off the shelf from various Vendors.
Lastly, you will likely need to do some Hammer and dolly work on the front of the transmission tunnel to clear the Valve covers, and allow them to be easily removed for service, etc. Not too much, just enough for this clearance.
Yes, there are numerous, and endless other ways to skin this same cat, but the above is - IMO - the easiest, and least trouble, resulting in the same conclusion.
Note: If the car originally had a 6 cyl., you are probably alright in the coil spring area, if it was a V8, you will need to replace the front coil springs, There are numerous threads and debates on this, and I don't know what your plans are for final height, lowered or raised, so this is another discussion to have.
Have fun!
Edit: Almost forgot: If you cut back and capacity extend the Oil Pan downward for steering linkage clearance, you'll need a longer oil pump pick-up. These are available from many vendors, I bought mine from Speedway Motors. I also use a two quart Spin-on oil filter (Wix) that helps in addition to aforementioned modifications to eliminate any overheating issues.
