you need to do what people call "clearancing" if the cam is new (a racing cam) then there are a number of locations that could contact rotating parts that should not.
Most times, the connecting rod will contact the area of the cam between the cam lobes, so the cam needs to be ground off at that spot.
Another spot is with these higher duration cams, the cam lobes contact the high edges of the crankshaft journal. The edges need to be ground down.
Other areas that contact sometimes are the cam lobe to connecting rod.
Lifters can top out on the flat spots of the lifter.
cam lobes can touch the aforementioned parts, as well as the side of the crank and connecting rod.
lobes of the cam can also touch the compression release and break it. It is important to be able to reach into the crankcase and actuate the compression release at all degrees of engine rotation through all cycles. You can have a compression release CLEAR everything when it is in its default position, but be in contact with rotating parts when it is in the other position.
you need to buy a side cover and cut the top and bottom off of it, so you can see into the crankcase from the bottom and top with a flashlight and turn the engine over. The crankcase cover will still need to be able to be bolted to the block, and just holds the cam and crank bearing.
good luck. If you just throw it together like that something will eventually break off, even if you can get it to run.
Also, since y ou said you are having that problem with the OLD cam and the NEW rod? Then it further shows that you might just have the connecting rod rubbing on the spot between the cam lobes. Also, check your crank end play as you tighten the crank. You might not have the crank bearings properly seated, which could bind the crank.
Bottom line, anything you touched could be messed up and you have to go through every thing you touched to rule out user error.
