In Stock cars, they work at 8,000+ rpm with huge roller cams, so they are strong, but they also get replaced regularly.
I think if you tone down the cam and springs and keep the RPM's sensible, they would probably last on the street.
The roller bearings and push rod adjuster cups are the part to worry about. Bigger springs and higher rpms will increse the forces on the roller bearing.
If you have big springs and high ratio rockers, the force exerted on the pushrod is very high. If it's too high, the valves that are in the open position when you stop the engine will push all of the oil film out of the adjuster cup and on startup the pushrod tip can gall. This is more pronounced on roller lifters because they don't spin like a flat tappet.
In Nascar, they pre lube before they start up an engine unless it was just recently running, so they avoid the problem.
I don't know if Jesel's are worth the money. There are some less expensive pieces that might be just as strong. If you keep your valve springs light to avoid the pushrod problems, you won't need all of that strength either.
Jesel has a Sportsman Series that retail for $695 almost anywhere. I just picked up a previously owned, but new set for $500, but I have been watching for them very intently for the last 5-6 months. If you haven't already, read the post I made to end out my thread here.
As luck would have it, I was also looking at a set like the ones you describe. The sellar says they have every option Jesel has, plus the setup tools and instructions. If you don't mind a used set for around $700-$800 check out this NMRA post. This guy has been asking $900 for them but it sounds like he is getting a little desperate; I suspect he needs money quickly. I wouldn't offer him over $750 to start with. They may be used, but they should still be way better than stud or pedestal mount rockers.
PS On Rev's comments. He is absoltely right about NASCAR changing them frequently, BUT they run 1.9 to well over 2.0 ratio rockers also and don't take ANY chances. You wouldn't want that high a ratio and they are available in more reasonable 1.5-1.7 ratios. With the kinds of springs you will run on the street, they will last longer than anything else out there except T&Ds maybe. But there are cheaper alternatives that are good enough for what we would typically put on the valve train, unless you are going to race Le Petite LeMans or GrandAm with your '40. As to pre-oiling, you can bet your butt that I am putting an Accusump on my car: the cheapest insurance for your motor there is. If money is an issue, don't buy a new one. Buy a used one for 1/10th the price and put a $8 O-ring kit in it. If you want a switch control, don't buy Tilton's or Moroso's; you can get a 12V solenoid valve for 1/3 what they want for them that is as good or better. Either way, if you are putting the kind of money in your engine that considering Jesel Mowhawks or Pro-Series rockers would indicate, it is a no brainer on the Accusump. If you don't do Jesel,T&D, Crower or the other $1K and up rockers, I would stay away from CAT and Probe for sure. Other than these, they are all pretty much the same price: ~$700 retail. So Jesel and Yella Terra are the best two IMHO with a bunch in the middle and Harland Sharp at the low end.
This is a little off topic, but I plan on doing pre lube a bit differently.
There are several compact electric pumps that can handle oil as well as the 300+ degree temps that an overheating engine might see.
With an extra pickup welded to the pan and a check valve, it should do a good job and you won't have to worry about an accusump bleeding down if you park it too long.
I'll also add a pressure switch so that the starter is locked out if I don't have 10-20 psi in the oil system. I might even have another pressure switch to turn on the pump if I loose oil pressure, that might save an engine if the oil pump drive shaft breaks.