Number 1,subframing is dangerous and doesn't work that well if whoever is doing the work doesn't truly understand all the engineering that goes into factory suspensions,and is a excellent welder with good fabrication skills..
Number 2 is that not everybody wants a old car that drives and rides like a new car. Some people prefer old cars that ride and drive like old cars because it brings back memories. Which is the reason many of us fool with old cars to start with.
A older car with a newer drivetrain and original suspension will ride and handle exactly like it did with the original drivetrain in it,but have more power. The rear end makes no difference at all on anything other than the engine rpms,and a 3:70 rear gear ratio (for example) in a 49 Chrysler acts no differently than a 3:70 rear drive ratio in a 2000 Ford Explorer.
I own old cars set up both ways. My 39 Ford has the original beam front axle with 4:10 Detroit Locker rear,automatic transmission,and probably 350 hp. The rear springs were changed to parrarrel springs (like your 49 Chrysler has) when the rear was installed. I am in desperate need of either a 4 bar setup or traction bars on the rear if I try to nail it hard taking off,but other than that I am happy with it in every respect.
My 48 Plymouth coupe has a 78 Camaro frame clip,motor,transmission,and rear end,and the entire suspension has been rebuilt. It rides and drives like a brand new 78 Camaro. It clearly handles better than a stock 48 Plymouth in the curves,but my Roger Road Racer days are over with,so that doesn't matter to me. I bought the car with the basic suspension install done,but had to finish it up and detail and rebuild it to get it to where it is today. If I had to do it again,I wouldn't. My OPINION is that it is just too damn much work for so little "real" return. These cars rode and drove fine as they left the factory,and there is no need to fix it if it ain't broken.
Now,if you have a shop,all the tools you need,and the experience to set it up and do it properly and you just want to do it,go ahead. Nothing wrong with it if that's what you want to do.
If this is your first project car and your tools and experience are limited,keep it simple,safe,and reliable. The goal is to get the car on the road so you can drive and enjoy it.
If you do decide to let him do a frame clip,make sure you get the names and phone numbers of other customers he has done this for,and check with them on the quality of his work and how happy they are with the end results.
If he can't or won't provide that,don't get him to do it.