I've done the swing to IRS conversion twice, albeit not on a speedster, but on Karmann Ghia's. I think the handling is far superior and worth the undertaking.
The pictures are from my 1958 LowLight Ghia. I also narrowed the control arms 1 5/8" per side to fit a larger tire under the fender.
I threw together a jig off a bare IRS chassis. You can buy these now from several places to make the conversion easier. A jig is absolutely necessary to get the proper orientation of the IRS bracket. Take your time when fitting the brackets to get a tight fit for clean welding.
I triangulated the rear section for strength and future attachment for a roll cage. I incorporated new shock mounts in the cross bar.
Those a type 2 CV joints, as they are larger and stronger than type 1. Axles were 16" that were narrowed 1 5/8" to fit the narrowed control arms.
This is how you fit a 205/65R15 under the fender of a Ghia. I think a 215/60 would have fit without issue.
Go for it Arden!