I was wondering what type of TH400 the 425's are matched with. I want to buy a shift improver kit but I dont know if its the TH-400 (3L80) or the TH-400/375/M40. Basically what kit do I buy of jegs? Also the 425 i've got is getting a cam in it, does jegs sell springs that'd be adequate for a .490 lift cam, or do I even need springs?

You should probably ask Jegs which kit would be best. There's a way to actually do the same thing the kit does without buying anything, I don't know exactly what the deal is but you might wanna search.

As for springs, your car has how many miles on it? The spring been compressed and uncompressed billions of times, new ones is a real good idea.

What are the dimensions and stiffness(lbs) of 425 valve springs though?