Use of doubler plates for attaching pipe covering to boiler cladding

Hi all - I am having problems in attaching a set of 8BA doubler plates to the inside surface of a 5"G locomotive boiler cladding. The general idea is to turn up a set of ~2mm thick brass discs tapped 8BA to provide say a 5X thread pitch depth for the fixing screws but the problem is just how to go about fixing them in position. Initial attempts using 60:40 soft solder is proving to be a bit of a disaster as it is difficult to a) initially set them into position and b) to keep them in position whilst attempting to solder in the neighbouring plate. I have tried some of the usual dodges such as using rusty old screws as a temporary fix during the soldering and have also tried typist correction fluid but given the relatively small sizes involved nothing has yet proved to be the answer. My question is therefore - Am I going about this in the right way or are there better alternatve methods, perhaps using adhesives in place of soft solder? I would be very interested to hear how other members have gone about this problem as I seem to have hit a brick wall. All suggestions would be most welcomed.

Tin the steel first, a 40w soldering iron will do. Or heat up a spare piece of copper or brass in a blowtorch flame and use that as an iron. Scrub it on the patch around each hole. Tin the brass disc's the same way. Align the brass disc's over each hole and apply heat to the disc and it will bond to the cladding.

I had a similar problem with the hood on a diesel loco which had a raised vent cowl across the top. I soldered 7BA brass nuts to the underside of the hood to take the fixing screws. It was 1.2mm galvanised steel sheet hood so relatively easy to solder. Just used my normal 40W electric soldering iron and Bakers flux with standard electrical solder. Make sure both surfaces are scrupulously clean though. I used wooden sprung clothes pegs to hold them in place as they don't act as a heat sink like metal clamps would.