Acording to those that I belive (Gene Berg and John Maher to name two) they're fine on the Type 3 engines that they originated on but not as good as the little standard tins on a Type 1. You can't really see them very well when they're fitted anyway. Has your rebuilt engine not come back with the pushrod tubes fitted/ valve gear set up etc. anyway?
Just make sure that the engine builder has fitted the deflector plates, they clip on under the cylinders, you can't fit them (or the alternative 'cool-tins') once the pushrod tubes etc. are in place.
I looked into these some time ago and so many different views about them. I did look at some that were not fitted and the quality was very poor (Cool air supplied ones). So decided not to bother and went down the colour coded push rod protectors... though I dont remember seeing them in Ginge....