MDC

The front triangle's normally held on with pins, so there shouldn't be any structural reason you can't do it. Could either just pick a free float rail that's shorter than your front sight, or one of the ones that has a cutout - e.g. https://madbull.com/catalog/en/products/handguard-ras-rail-mount/daniel-defense-licensed-omegax-rail-12-fsp-fde-tan.html

Personally I'd pick the Blaster's, however if you'll be buying these online (rather than in a site shop) then ASG sell their "Cursed" BB range and they do a 0.28g in this (for example a 1kg bag of them from Patrol Base). My son, and myself occasionally, have been using Cursed 0.28g BB's for ages and they've always been faultless but they're so cheap if you buy them in bulk from Patrol Base (especially if you can save nay other purchases for the same time like some gas).

ASG Devils/Blasters are pretty much my go to choice, theyre cheap and never had any issues with them. 
 
 

Same here; they are decent BBs at a decent price.

Devil's. I've used them for ages before going over to geoffs.

Never had a problem with devils either but tbh I tend to just buy whatever's on sale at the site

Just to check, those appear to be non-bios. Is that what everyone is talking about?  I've had no problem with Vorsk 0.28g bios. I've not used the ASG devils in that weight, bio or otherwise, but their 0.25g non-bios are fine.

My go to choices are Geoff's and Warhead, but I've also used 'BB King' from Spec Ops and they've been fine 

I’ve fitted an XT100 from AK2M4 today. It’s now bang on the money at 348/349 with .20g.  Thank you for your advice and help fella.👌🏻

That is great news

I’ve fitted an XT100 from AK2M4 today. It’s now bang on the money at 348/349 with .20g.  Thank you for your advice and help fella.👌🏻

I’ve fitted an XT100 from AK2M4 today. It’s now bang on the money at 348/349 with .20g.  Thank you for your advice and help fella.👌🏻

I’m going to order a XT100 and a XT90👍🏻

I’m going to order a XT100 and a XT90👍🏻

Mmm, airsoft spring ratings are more what you'd call guidelines than actual rules. The good news is that it sounds like you've got great airseal, so you can downspring and every other component will thank you for it.
 
While I'll stress that @ak2m4 really does know what he's talking about, it is just possible that you might need to go lower than M100 on an XT spring.  I absolutely fluked out on a short-tight-barrel M4 and hit 1.3J with an M100 XT spring and had to drop to an M90 to tame it.  But that's a freakish occurrence, and an XT M100 will typically be the sweet spot for UK guns.

Really depends on how they rated their madbull m100 springs, I think they rate them with 0.25g.  They do give a kg rating though which is pretty useless.  For the XT springs grab a M100, then you can always adjust from there once tested 

It’s a Ultimate/Lonex gearbox. I opened it and fitted the Mad Bull 100 spring years ago. It then just sat in a gun case for 20 plus years…I recently refreshed it with a maple leaf barrel and purple Prometheus bucking. I converted it to Lipo too from Tamiya. It’s a punchy little gun and I really want to use it..I guess a 90 spring would do it?

First off, if you built this RIF 25+ years ago, have you checked inside it for any issues? Is the hop rubber still fresh and soft?
 
Second, what ammunition are you using? BB quality is a big deal.
 
Third, you shouldn't ever need PTFE tape on an AEG. That mod originated for VSR cut barrels and hop rubbers, where the design allows air to balloon the hop packing and cause a leak. If the hop unit is loose on the rubber, then yes, you should wrap the rubber/barrel in layers of tape for stability, but that's a spec issue at heart, and I would suggest finding a rubber or unit that fits better.
 
Fourth, this issue could be something as simple as the nub or hop arm having shifted, putting more pressure on one side of the rubber. In order for us to give you better advice, I'd suggest disassembling your barrel group, testing and feeling everything, and post pictures! Give us the details and the nitty gritty.

Yes, I am aware it’s running hot hence not used it at AP. 

Yes, I am aware it’s running hot hence not used it at AP. 

It’s correctly fitted, I’ve even wrapped ptfe tape around hop unit to barrel for a proper air seal.