In my T4, a 1992 converted by Westfalia, there are two seatbelts installed for use by passengers seated in the rear seats - the seats that fold down to make the bed. One seat belt is a shoulder belt, the other, next to the cabinetry, is a lap belt only. Rear tethers are attached to provided mount points beneath the rear roof mounted cabinet. Both seat belts can be used (they are plenty long enough) to secure a modern child or infant seat. Most modern child/infant seat systems, including Isofix, are designed so they can also be installed using seat belts instead.
I struggle to visualize a child/infant car seat that could not be installed using my belts and tethers - the ones Westfalia put in my van as OEM. If the belt can be worn by an average size person, they are able to hold a child/infant car seat by default.
You mention your van has been converted. Do you mean converted by a previous owner or yourself? Perhaps the conversion did not properly accommodate proper/OEM seat belts in the rear? Is your child/infant car seat made to only install using the Isofix or similar attachment system and not work with seat belts?
Also, you say the belt won't fit "around" the baby's car seat. The infant (rear facing) seats usually have the belt going through the base, not around the unit. Is that not the case with yours? Then the seat/carrier part snaps onto the base. On some designs there is a support from the back of the carrier, pointing forward to the front of the vehicle, bracing against the floor as well.
Anyway, good luck sorting this out. There should be a safe, legal solution, and congrats having a four month old - and a T4!