340 help

For you,I would skip on the 340 original stroke and go straight to a 360.
In a DD or streeter you can run less cam and get more low speed torque production out of it with all other parts equal. But when the compressions are optimized for the cam,in that 360, you will get even more bottom end torque. This lets you run less gear,for hiway cruising. And for me the clincher was fuel-mileage. With the ever-rising cost of fuel, the 360 can be engineered to deliver incredible economy, yet still make a lot of power.
Yeah sure, either can be engineered to have the same power, but this is IMO,only good for racers. On the street power at 6000 is not always what you want. Sometimes just getting off the line briskly is, and 4.10s are not usually on the table.
Some say compression is not that big of a deal. And for most that might be true. But for a hot streeter, I disagree. The more you run the better. Throttle tip-in, the low-speed performance,Torque in the midrange, and fuel economy will all benefit with the compression maxed out. As to power, it's true that there is only marginal difference between 10/1 and 9.5/1. But I will never again build a less than optimum engine for myself.
By having your very own 360 core, you can engineer it from day1, to suit your useage. And when your A/F-body rusts to dust, you can pull the 360 out and stick it into a truck for many more years,lol.
If you have a choice, a Magnum roller is the thing to get.And closed chamber heads. You can also run more compression with aluminum heads too; so there is that as well.
so my vote goes to a 5.9Magnum.