it's the head that's your main issue in this situation.... i.e. stay away from the beltex/texel stuff as they have *massive* heads.
It's not the head. People get hung up about heads, because when lambs get stuck, the head swells, making it hard to push back in to sort out the legs. I've had heads too large to push back in in pure Swaledale matings, due to this swelling up.
The only breed in which the head can be an issue, as far as I know, is something like an Exmoor Horn, where the unborn male lambs already have significant horn buds. In a good single tup lamb, the bony width of the horned skull can be an issue - hence why Exmoor Horn breeders always strive for twins.
With Beltex and Texel lambs, the significant width is at the shoulders and sometimes also the buttocks. However, frankly, if the ewe is of more native or primitive breeding, there is much less likelihood of there being an issue. Partly because the ewe has no internal double muscling, and partly because the crossbred Texel lamb hasn't quite the conformation of a more purebred one.
All of which said, I'd hesitate to use an unknown Texel or Beltex tup on a first-timer of any breed, except perhaps a Swaledale Mule. Even the pedigree Beltex breeders have been known to use a Shetland tup for the first mating, to get easily-lambed, active lambs that know how to find the milk bar