Wondering if it makes sense to buy my own tool to test my valve springs. Or pay an engine shop to do it? The tool would probably get used on one engine only.

The FSM specs for 340 says to test loads both closed and open, 80-90 lbs @ 1-11/16" and 235-249 lbs @ 1-7/32".

Cheapest tools I've found are like this one for $80:

http://www.racingjunk.com/Engines-Components/2458778/VALVE-SPRING-TESTER.html

But I don't think my Harbor Freight torque wrench will go to 235 lbs.

Any thoughts on this appreciated. Thanks.

I assume you have a hydraulic cam ?
Pay a good shop to test the load rate of closed and open pressures. Buy a spring with a closed rate of 100-105 lbs and a open rate close to 280lbs, at your installed spring height from the spring seat to the bottom of the retainer. just my opinion for better valve control. With stock 3/8 OD stem heavy valves the extra spring load is needed and will not hurt the cam with good oil.
How close are you to South Houston Engine ? They are a good shop. I've been there on business a few times for Radiac abrasives and they can do the job.

MLR426