question on 340 to 360 swap

727 and 904 converters don't interchange.

If the converter bolt holes match up on the OEM plates & converter you will use, you can use the same one, but with a 360 LA 727 converter. (There were different bolt diameters and locations over the years. But I'm not familiar with the details. The B&M flexplates have the differences covered.)

If your 340 balancer/damper is flat, it's a forged crank and internally balanced. You can use a B&M LA 360 flexplate with your 340 converter.

If your 340 has a cast-in counterweighted ring on the balancer/damper that says "FOR 340 CAST CRANK ONLY", or something like that, it is externally balanced. You'll need a 360 specific 727 converter with neutral flexplate or a neutral 727 converter with a 360 LA specific flexplate.