The main problem is the oil we use today. I had a small block we ran in a dragster with solid tappet flat tappet cam with EDM lifters and because he did not use Zinc additive in the race oil the cam was gone in 50 passes. The oil advertised having zinc, but not enough. The cam was broken in on my run stand and all the cam break in procedures were followed. Plus what you are giving up in performance it does not make sense to use a flat tappet cam in today's world. I do almost all stroker motors both big block and small block MOPAR and I do not use flat tappet cams.
The reason they are telling it will run on pump gas the cam is bleeding off compression pressure in the cylinder because the duration at.050. The motor under 3000 will be slow, but above that it will come alive. This purely a race cam. With the converter you stated that it will work with that cam so it will idle at about 1200 RPM at a stop light. Now the downside you will need to get the biggest trans cooler you can find because it will never lockup under 4000 rpm and it will generate a ton of heat. This because it will be always slipping. Been down this road before.
I used FELPRO gaskets for 30 years mostly because there was nothing else and after some time I would see the same failures between the cylinders. For the last 8 years I have used Cometic multi layer gaskets and have never had a failure. In one instance do to a internal oil leak I reused the gasket 4 times and it lived the entire race season.
You did not state what you are using for ignition, carb, fuel system, rear gear, rear tire size. Are you installing a trans brake? How much time on the track or on the street. These are some other items to consider when working out a combination for the car. Exactly what do you want the car to do and will this combination you have so far get you there.
Oh by the way my 340 is actually 430 inches in a '70 block.
