Here's a bare CMC sub-frame from a burnt up Speedster (ignore red marks). The square tubing to the rear is not very strong and relies on a few rivets to it to support the tail fiberglass (hence butt sag with a bigger gap at rear top of doors (easy fix). I like the way you moved it back. With a T1 engine the transmission/frame horns and an add-on Kaffer brace will handle a good size engine (if frame horns don't have internal rust). The rear brace would be handy for hanging a heavier type 4 engine or a Subaru rear mounting point.
Without door strikers installed, will your door close further? Mine are only stick out for a few inches up the door (maybe 4-5 max). Or do you have rubber seal installed pushing it out? Alan for sure has an idea.